System, a method and a computer program product for maneuvering of an air vehicle

ABSTRACT

A control system configured to control an acceleration of an air vehicle which comprises a tiltable propulsion unit that is tiltable to provide a thrust whose direction is variable at least between a general vertical thrust vector direction and a general longitudinal thrust vector direction with respect to the air vehicle, the control system comprising: (a) an input interface for receiving information indicative of a monitored airspeed of the air vehicle; and (b) a control unit, configured to issue controlling commands to a controller of the tiltable propulsion unit for controlling the acceleration of the air vehicle.

FIELD OF THE INVENTION

This invention relates to systems, methods and computer program productsfor maneuvering of an air vehicle.

BACKGROUND OF THE INVENTION

While the concept of implementing helicopter-like rotors on a wingedaircraft can be traced back to as early as the 1930s, actual productionof such aircraft took much longer to mature. The experimental Bell XV-3aircraft, which was built in 1953, proved the fundamental soundness ofthe tiltrotor concept, and gathered data about technical improvementsneeded for future design.

A tiltrotor is an aircraft which utilizes one or more powered rotors(sometimes called proprotors) mounted on respective one or more tiltablestructures. Those rotors may be used for both lift and propulsion,wherein different behavior is achieved when the rotors are tiltedbetween a general vertical direction and a general horizontal direction,and in intermediate directions. Tilting of the rotors enablesimplementing, at certain times, vertical lift capability usuallyassociated with helicopters, and at other times, the propellerpropulsion usually associated with conventional fixed-wing aircraft. Itis noted that while in tiltrotor aircraft usually only the rotor (and astructure onto which it is mounted) is usually tilted, tiltwing aircraftwere also developed in which the entire wing—including the one or morerotors mounted thereon—is tilted.

Experiments in the field continued in the 1970's and 1980's with thedevelopment of the XV-15 twin-engine tiltrotor research aircraft,followed by the developing by Bell and Boeing Helicopters—starting in1981—of the “V-22 Osprey” aircraft, which is a twin-turboshaft militarytiltrotor aircraft for military needs. Another aircraft implementingtilt rotor technology is the commercial BA609 aircraft of Bell, teamedwith AgustaWestland.

Tiltrotor unmanned aerial vehicles (UAVs) were also developed in the1990's and 2000's, such as Bell's TR918 Eagle Eye, and IAI's PantherUAV.

By way of general background, the following publications disclosevarious aircraft configurations.

U.S. Pat. No. 7,267,300 discusses an aircraft comprising an airframe, apower plant mounted on the airframe, and at least two propellersrotatably mounted on the airframe and powered by the power plant formoving the aircraft in a generally forward direction during operation ofthe propellers. Also, the aircraft includes at least twocounter-rotatable fan sets mounted on the airframe and powered by thepower plant for providing upward lift to the aircraft during operationof the fan sets.

US 2006/0226281 discusses a vertical take-off and landing vehiclecomprised of a fuselage having a front, a rear, and two lateral sidesand a set of four thrusters set to the front, the left, the right, andthe rear of said fuselage. The thrusters are comprised of a set of twocounter rotating propellers both of which creates lift. The two counterrotating propellers cancel out the torque effect normally created byusing only one propeller. The ducted fan units are movable between afirst position in which they provide vertical lift and a second positionin which they provide horizontal thrust using a set of servos and gears.

U.S. Pat. No. 7,472,863 discusses a vertical takeoff and landing (VTOL)aircraft design comprising one internal combustion engine able to spitshaft power to four fan units.

The fan units further employ counter rotating fan blades for stability.Separate horizontal and vertical tilting mechanisms delivered to the fanunits are additionally disclosed. A variation in design is furtherincluded wherein electric motors provide the necessary shaft power.

US 2004/094662 discusses an Unusual Flying Object said to have VTOLcapabilities including forward flight with a Linear Induction MagneticBearings power drive.

U.S. Pat. No. 7,461,811 discusses a STOL or VTOL winged aircraftcomprising a fuselage and a fixed wing attached to the fuselage andextending outward from the two lateral sides thereof, forming one wingcomponent extending outward from one side of the fuselage and a secondwing component extending outward from the opposite side of the fuselage.At least one “thruster” is disposed in each wing component to providevertical lift to the aircraft when the aircraft is stationary or movingforward only slowly. The thruster includes a shaft mounted for rotationin the respective wing component and extending substantially parallel tothe wing axis and a plurality of fan blades attached to the shaft formovement of air.

US 2003/062442 discusses a personal aircraft said to be capable ofvertical take-off and landing and comprises a passenger compartmenthaving a front, a rear and two sides, and a plurality of independentlypowered thrusters attached to the outer periphery of the compartment. Atleast three thrusters are disposed on each side of the compartment. Thethrusters, which are preferably ducted fan units, are capable ofproviding a vertically upward force to the compartment.

U.S. Pat. No. 6,892,979 discusses a personal aircraft said to be capableof vertical take-off and landing which comprises: (a) a fuselage havinga front end, a rear end and two lateral sides, the fuselage having acentral longitudinal axis extending from the front end to the rear end,between the two lateral sides; (b) at least one, and preferably two ormore, ducted fans, each arranged in the fuselage between the front endand the rear end and between the two lateral sides, for providingvertical lift; and (c) at least one substantially horizontal wingattached to each side of the fuselage and extending outward with respectto the central longitudinal axis.

U.S. Pat. No. 6,464,166 discusses a vehicle, particularly a VTOL airvehicle, including a duct carried by the vehicle frame with thelongitudinal axis of the duct perpendicular to the longitudinal axis ofthe vehicle frame; a propeller rotatably mounted within the duct aboutthe longitudinal axis of the duct to force an ambient fluid, e.g. air,therethrough from its inlet at the upper end of the duct through itsexit at the lower end of the duct, and thereby to produce an upward liftforce applied to the vehicle; and a plurality of parallel, spaced vanespivotally mounted to and across the inlet end of the duct about pivotalaxes perpendicular to the longitudinal axis of the duct andsubstantially parallel to the longitudinal axis of the vehicle frame.The vanes are selectively pivotal to produce a set horizontal forcecomponent to the lift force applied to the vehicle. Various vanearrangements are disclosed for producing side, roll, pitch and yawmovements of the vehicle.

US 2003/080242 discusses an aircraft that is mounted with turbofanengines with separate core engines having fan engines used commonly forcruising and lifting up, through enabling to direct the thrust from fanengines to all directions by supporting the fan engines composing theturbofan engines with separate core engines in biaxial support so thatthe fan engines are rotatable in the direction of pitching and rolling,the fan engines are mounted on both sides of each of front and rearwings.

US 2007/0057113 discusses a system and method provided for a STOL/VTOLaircraft that stores required take-off power in the form of primarily anelectric fan engine, and secondarily in the form of an internalcombustion engine.

US 2008/0054121 discusses a VTOL vehicle comprising a fuselage havingforward and aft propulsion units, each propulsion unit comprising apropeller located within an open-ended duct wall wherein a forwardfacing portion of the duct wall of at least the forward propulsion unitis comprised of at least one curved forward barrier mounted forhorizontal sliding movement to open the forward facing portion tothereby permit air to flow into the forward facing portion when the VTOLvehicle is in forward flight.

US 2002/113165 discusses a vertical takeoff aircraft that uses ductedfans for lift and propulsion. The fans are attached to an airframe andare disposed on opposite lateral sides of the aircraft. The thrust fromthe each of the fans may be deflected in different directions by usingvanes with flaps disposed within the ducts of the fans, as well as bytilting the entire fan assemblies.

U.S. Pat. No. 6,488,232 discusses a single passenger aircraft configuredto vertically take-off and land. An airframe is configured to supportthe passenger in an upright position during take-off and landing andduring flight. The aircraft includes a pair of propulsion devices thatare mounted on an airframe above the level of the pilot. A set of handoperated control devices are mechanically linked to the propulsiondevices for varying the orientation of the propulsion devices duringflight.

WO 2010/137016 discusses a system and method for providing propulsionand control to an air vehicle, and for operating the vehicle, in whichat least three propulsion units provide vertical thrust for vectoredthrust flight, and in which at least one or two of the propulsion unitsalso provide thrust for vectored thrust cruising or aerodynamic flightby suitably tilting the respective propulsion units for changing thethrust vector thereof. At the same time, the three or more propulsionunits are operated to generate controlling moments to the air vehicleabout three orthogonal axes, pitch, roll and yaw, during vectored thrustflight (hover, cruising, etc.) or during aerodynamic flight forcontrolling the vehicle. The control moments are generated byselectively varying the thrust generated by each of the propulsion unitsindependently of one another, and: by selectively vectoring the thrustof one propulsion unit with respect to each of two independent tilt axesindependently of one another, or by selectively vectoring the thrust ofeach of two propulsion units with respect to a respective tilt axis,independently of one another.

WO 2008/054234 discusses a propulsion system of a vertical takeoff andlanding aircraft or vehicle moving in any fluid or vacuum and moreparticularly to a vector control system of the vehicle propulsion thrustallowing an independent displacement with six degrees of freedom, threedegrees of translation in relation to its centre of mass and threedegrees of rotation in relation to its centre of mass. The aircraftdisplacement ability using the propulsion system of the presentinvention depends on two main thrusters or propellers which can betilted around pitch axis by means of tilting mechanisms, used to performa forward or backward movement, can be tilted around roll axis by meansof tilting mechanisms, used to perform lateral movements to the right orto the left and to perform upward or downward movements, the mainthrusters being further used to perform rotations around the vehicle yawaxis and around the roll axis. The locomotion function also uses one ortwo auxiliary thrusters or propellers mainly used to control therotation around the pitch axis, these thrusters or propellers and beingfixed at or near the longitudinal axis of the vehicle, with their thrustperpendicular or nearly perpendicular to the roll and pitch axis of thevehicle.

Israeli patent application 217,501 discusses a control system configuredto control a deceleration process of an air vehicle which comprises atleast one tiltable propulsion unit, each of the at least one tiltablepropulsion units is tiltable to provide a thrust whose direction isvariable at least between a general vertical thrust vector direction anda general longitudinal thrust vector direction with respect to the airvehicle

There is a need in the art for an air vehicle that can descend to ahover and for systems, methods and computer program products formaneuvering of air vehicles, and especially of tilt-rotor air vehicles.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is disclosed a controlsystem configured to control an acceleration of an air vehicle whichincludes a tiltable propulsion unit that is tiltable to provide a thrustwhose direction is variable at least between a general vertical thrustvector direction and a general longitudinal thrust vector direction withrespect to the air vehicle, the control system including: an inputinterface for receiving information indicative of a monitored airspeedof the air vehicle; and a control unit, configured to issue controllingcommands to a controller of the tiltable propulsion unit for controllingthe acceleration of the air vehicle, wherein the controlling of theacceleration includes: (1) in a first part of the acceleration, in whichthe tiltable propulsion unit provides thrust in the general verticalthrust vector direction: (a) controlling an operation of the at leastone tiltable propulsion unit for providing lift to the air vehicle, and(b) controlling a modifying of a tilt angle of the tiltable propulsionunit with respect to a fuselage of the air vehicle, based on: (i) themonitored airspeed and (ii) an airspeed command; and (2) following atilting of the at least one tiltable propulsion unit, controlling in asecond part of the acceleration an operation of the tiltable propulsionunit to provide thrust in the general longitudinal thrust vectordirection for propelling the air vehicle.

Optionally, the control unit may be configured to control the operationof the tiltable propulsion unit automatically.

Optionally, the control unit may include a pitch control module which isconfigured to issue a pitch command based on the monitored airspeed andthe airspeed command and to control in the first part of theacceleration a modifying of a pitch angle of the air vehicle based onthe pitch command, wherein the controlling of the modifying of the tiltangle is further based on the pitch command.

Optionally, the control unit may be configured to issue a rotor tiltcommand for lowering a rotor tilt angle as a result of an issue of apitch command for lowering a pitch angle.

Optionally, the pitch control module may be configured to issue thepitch command further in response to the measured altitude of the airvehicle.

Optionally, the control unit may be configured to change a ratiobetween: (a) a contribution of the tiltable propulsion unit to themodification of the pitch and (b) a contribution of at least oneelevator to the modification of the pitch, wherein the changing of theratio is correlated to the monitored airspeed.

Optionally, the control unit may be further configured to control thrustpower of the tiltable propulsion unit for reducing a difference betweenthe measured airspeed and a set airspeed, while restricting increasingof the thrust power based on a threshold that is determined in responseto the measured airspeed.

Optionally, the control unit includes a tilting control module that isconfigured to determine a timing for tilting of the at least onetiltable propulsion unit between the second and the first parts of theacceleration for minimizing a duration in which the tiltable propulsionunit provides thrust in the general vertical thrust vector direction.

Optionally, the pitch control module may be further configured to keep apitch of the air vehicle during at least a part of the first part of theacceleration within a permitted pitch range that is dynamicallydetermined in response to the measured airspeed of the air vehicle

Optionally, the control unit may be configured to control before thefirst part of the acceleration a substantially vertical ascent of theair vehicle from the ground, and to control aerodynamic parts of the airvehicle in a first part of the substantially vertical ascent from theground for minimizing change in pitch angle and roll angle of the airvehicle.

Optionally, the control unit may be configured to issue a positiveairspeed command for accelerating the air vehicle if predetermined timeelapsed from an initiation of the substantially vertical ascent,regardless of sensors data.

Optionally, the control unit may further include a pitch control modulewhich is configured to control in the first part of the acceleration amodifying of a pitch angle of the air vehicle, wherein the control unitis configured to balance between modifying the tilt angle and modifyingthe pitch angle for reducing an energy cost of the acceleration of theair vehicle.

Optionally, the control unit may further include a pitch control modulewhich is configured to control in the first part of the acceleration amodifying of a pitch angle of the air vehicle, and to control in thefirst part of the acceleration a modifying of a pitch angle of the airvehicle, thereby preventing creation of negative lift force by the wing.

Optionally, the control unit may further include a pitch control modulewhich is configured to control in the first part of the acceleration amodifying of a pitch angle of the air vehicle and to limit in the firstpart of the acceleration lift created by the wing below 80% of maximallift which may be created by the wing in the monitored airspeed.

Optionally, at least the controlling of the accelerating may includecontrolling an operation of at least one aerodynamic part of the airvehicle selected from a group consisting of an aileron, an elevator, arudder, a ruddervator, a flaperon, elevons, and a wing flap.

Optionally, the control unit may be configured to control thrust powerof the tiltable propulsion unit for reducing a difference between themeasured airspeed and a set airspeed, while restricting reduction of thethrust power based on a lower threshold that is determined in responseto a measured airspeed of the air vehicle.

Optionally, the control unit may further include an altitude controlmodule configured to minimize a vertical deviation of the air vehiclefrom a set altitude during the first part of the acceleration, whereinthe minimizing is restricted at least by the restricting of thereduction of the thrust power based on the lower threshold.

According to an aspect of the invention, there is disclosed an airvehicle system, including: a wing; a tiltable propulsion unit, tiltableto provide a thrust whose direction is variable at least between ageneral vertical thrust vector direction and a general longitudinalthrust vector direction with respect to the air vehicle; and a controlunit, configured to issue controlling commands to a controller of thetiltable propulsion unit for controlling the acceleration of the airvehicle, wherein the controlling of the acceleration includes: (1) in afirst part of the acceleration, in which the tiltable propulsion unitprovides thrust in the general vertical thrust vector direction: (a)controlling an operation of the at least one tiltable propulsion unitfor providing lift to the air vehicle, and (b) controlling a modifyingof a tilt angle of the tiltable propulsion unit with respect to afuselage of the air vehicle, based on: (i) a monitored airspeed of theair vehicle and (ii) an airspeed command; and (2) following a tilting ofthe at least one tiltable propulsion unit, controlling in a second partof the acceleration an operation of the tiltable propulsion unit toprovide thrust in the general longitudinal thrust vector direction forpropelling the air vehicle.

All the variations discussed above with respect to the control systemmay also be implemented for the control unit of the air vehicle system.

According to an aspect of the invention, there is disclosed a method forcontrolling an acceleration of an air vehicle which includes a tiltablepropulsion unit that is tiltable to provide a thrust whose direction isvariable at least between a general vertical thrust vector direction anda general longitudinal thrust vector direction with respect to the airvehicle, the method including: (1) in a first part of the acceleration,in which the tiltable propulsion unit provides thrust in the generalvertical thrust vector direction: (a) controlling an operation of the atleast one tiltable propulsion unit for providing lift to the airvehicle, and (b) controlling a modifying of a tilt angle of the tiltablepropulsion unit with respect to a fuselage of the air vehicle, based on:(i) the monitored airspeed and (ii) an airspeed command; and (2)following a tilting of the at least one tiltable propulsion unit,controlling in a second part of the acceleration an operation of thetiltable propulsion unit to provide thrust in the general longitudinalthrust vector direction for propelling the air vehicle.

Optionally, the controlling of the modifying of a tilt angle of thetiltable propulsion unit includes automated controlling by at least oneprocessor of a control unit mounted on the air vehicle.

Optionally, the controlling of the acceleration of the air vehicleincludes controlling the acceleration of the air vehicle that includes awing, and balancing between contradictory aerodynamic effects resultingfrom the wing and from the at least one tiltable propulsion unit.

Optionally, the method may further include issuing a pitch command basedon the monitored airspeed and the airspeed command and controlling inthe first part of the acceleration a modifying of a pitch angle of theair vehicle based on the pitch command, wherein the controlling of themodifying of the tilt angle is further based on the pitch command.

Optionally, the controlling of the modifying of the tilt angle includesissuing a rotor tilt command for lowering a rotor tilt angle as a resultof an issue of a pitch command for lowering a pitch angle.

Optionally, the issuing of the pitch command is further based on themeasured altitude of the air vehicle.

Optionally, the method may include controlling thrust power of thetiltable propulsion unit for reducing a difference between the measuredairspeed and a set airspeed, while restricting increasing of the thrustpower based on a threshold that is determined in response to themeasured airspeed.

Optionally, the method may include determining timing for tilting of theat least one tiltable propulsion unit between the second and the firstparts of the acceleration for minimizing a duration in which thetiltable propulsion unit provides thrust in the general vertical thrustvector direction.

Optionally, the method may include keeping a pitch of the air vehicleduring at least a part of the first part of the acceleration within apermitted pitch range that is dynamically determined in response to themeasured airspeed of the air vehicle

Optionally, the keeping of the pitch within the permitted pitch rangeprevents stalling of the air vehicle.

Optionally, at least the controlling of the accelerating includescontrolling an operation of at least one aerodynamic part of the airvehicle selected from a group consisting of an aileron, an elevator, arudder, a ruddervator, a flaperon, elevons, and a wing flap.

Optionally, the method may include controlling thrust power of thetiltable propulsion unit for reducing a difference between the measuredairspeed and a set airspeed, while restricting reduction of the thrustpower based on a lower threshold that is determined in response to ameasured airspeed of the air vehicle.

Optionally, the method may include minimizing a vertical deviation ofthe air vehicle from a set altitude during the first part of theacceleration, wherein the minimizing is restricted at least by therestricting of the reduction of the thrust power based on the lowerthreshold.

According to an aspect of the invention, there is disclosed a programstorage device readable by machine, tangibly embodying a computerreadable code portion executable by the machine for controlling anacceleration of an air vehicle which includes a tiltable propulsion unitthat is tiltable to provide a thrust whose direction is variable atleast between a general vertical thrust vector direction and a generallongitudinal thrust vector direction with respect to the air vehicle,the computer readable code portion including instructions for: (1) in afirst part of the acceleration, in which the tiltable propulsion unitprovides thrust in the general vertical thrust vector direction: (a)controlling an operation of the at least one tiltable propulsion unitfor providing lift to the air vehicle, and (b) controlling a modifyingof a tilt angle of the tiltable propulsion unit with respect to afuselage of the air vehicle, based on: (i) the monitored airspeed and(ii) an airspeed command; and (2) following a tilting of the at leastone tiltable propulsion unit, controlling in a second part of theacceleration an operation of the tiltable propulsion unit to providethrust in the general longitudinal thrust vector direction forpropelling the air vehicle.

All of the variations discussed above with respect to the method may beimplemented as instructions of the computer readable code for theexecution of the relevant stages, processes, acts, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, embodiments will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIGS. 1A and 2A are front/top/side isometric views of a first embodimentand a second embodiment, respectively, of an air vehicle for which theinvention may be implemented when its tiltable propulsion units aredirected in a general longitudinal thrust position, in a first flightmode;

FIGS. 1B and 2B are front/top/side isometric views of the air vehiclesof FIGS. 1A and 2A respectively, when the respective tiltable propulsionunits are directed in a general vertical thrust position, in a secondflight mode;

FIG. 3A is a flow chart of a method for controlling a decelerationprocess of an air vehicle which includes at least one tiltablepropulsion unit, according to an embodiment of the invention;

FIG. 3B is a flow chart of a method for controlling a substantiallyvertical descent of an air vehicle, according to an embodiment of theinvention;

FIGS. 4A and 4B illustrate two possible flight courses of an air vehicleduring which it decelerates, according to an embodiment of theinvention;

FIG. 5 is a graph which illustratively exemplifies restriction of thrustpower based on the measured airspeed of the air vehicle, according to anembodiment of the invention;

FIG. 6 is a graph which illustratively exemplifies restriction of pitchof the air vehicle based on the measured airspeed thereof, according toan embodiment of the invention;

FIG. 7 illustrates a possible exemplary flight course of an air vehicle,during which it decelerates, according to an embodiment of theinvention;

FIG. 8 schematically illustrates a control system, according to anembodiment of the invention;

FIG. 9 schematically illustrates a control system, according to anembodiment of the invention, as well as its environment;

FIG. 10 is a flow chart of a method for controlling an acceleration ofan air vehicle which includes a tiltable propulsion unit that istiltable to provide a thrust whose direction is variable at leastbetween a general vertical thrust vector direction and a generallongitudinal thrust vector direction with respect to the air vehicle,according to an embodiment of the invention;

FIG. 11 is a graph which illustratively exemplifies restriction ofthrust power based on the measured airspeed of the air vehicle duringthe acceleration, according to an embodiment of the invention;

FIG. 12 is a graph which illustratively exemplifies restriction of pitchbased on the measured airspeed of the air vehicle during theacceleration, according to an embodiment of the invention;

FIG. 13 is a graph illustrating an airspeed command that may be issuedby a longitude airspeed management module, according to an embodiment ofthe invention.

FIG. 14 illustrates interrelations between various components of thesystem of FIG. 9, according to an embodiment of the invention.

FIG. 15 illustrates interrelations between various components of thesystem of FIG. 9, the system of FIG. 9, according to an embodiment ofthe invention.

FIG. 16 is a graph illustrating the measured altitude, the altitudecommand, the difference between them (dZ), and the issued RoC commandwhich is issued and used for the determination of the pitch command andtilt command, according to an embodiment of the invention;

FIG. 17 illustrates interrelations between various components of thesystem of FIG. 9, according to an embodiment of the invention;

FIG. 18 illustrates interrelations between various components of thesystem of FIG. 9, according to an embodiment of the invention;

FIG. 19 illustrates a possible control scheme which may be implementedby the system of FIG. 9, according to an embodiment of the invention;and

FIG. 20 illustrates a possible exemplary flight course of an air vehicleduring which it accelerates, according to an embodiment of theinvention.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, and components have notbeen described in detail so as not to obscure the present invention.

In the drawings and descriptions set forth, identical reference numeralsindicate those components that are common to different embodiments orconfigurations.

Unless specifically stated otherwise, as apparent from the followingdiscussions, it is appreciated that throughout the specificationdiscussions utilizing terms such as processing, calculating,determining, generating, setting, configuring, selecting, or the like,include action and/or processes of a computer that manipulate and/ortransform data into other data, said data represented as physicalquantities, e.g. such as electronic quantities, and/or said datarepresenting the physical objects. The terms “computer”, “processor”,“control unit”, “control system”, “control module” and the like shouldbe expansively construed to cover any kind of electronic device withdata processing capabilities, including, by way of non-limiting example,a personal computer, a server, a computing system, a communicationdevice, a processor (e.g. digital signal processor (DSP), amicrocontroller, a field programmable gate array (FPGA), an applicationspecific integrated circuit (ASIC), etc.), any other electroniccomputing device, and or any combination thereof.

The operations in accordance with the teachings herein may be performedby a computer specially constructed for the desired purposes or by ageneral purpose computer specially configured for the desired purpose bya computer program stored in a computer readable storage medium.

As used herein, the phrase “for example,” “such as”, “for instance” andvariants thereof describe non-limiting embodiments of the presentlydisclosed subject matter. Reference in the specification to “one case”,“some cases”, “other cases” or variants thereof means that a particularfeature, structure or characteristic described in connection with theembodiment(s) is included in at least one embodiment of the presentlydisclosed subject matter. Thus the appearance of the phrase “one case”,“some cases”, “other cases” or variants thereof does not necessarilyrefer to the same embodiment(s).

It is appreciated that certain features of the presently disclosedsubject matter, which are, for clarity, described in the context ofseparate embodiments, may also be provided in combination in a singleembodiment. Conversely, various features of the presently disclosedsubject matter, which are, for brevity, described in the context of asingle embodiment, may also be provided separately or in any suitablesub-combination.

In embodiments of the presently disclosed subject matter one or morestages illustrated in the figures may be executed in a different orderand/or one or more groups of stages may be executed simultaneously andvice versa. The figures illustrate a general schematic of the systemarchitecture in accordance with an embodiment of the presently disclosedsubject matter. Each module in the figures can be made up of anycombination of software, hardware and/or firmware that performs thefunctions as defined and explained herein. The modules in the figuresmay be centralized in one location or dispersed over more than onelocation.

The herein disclosed systems, methods, and computer program products maybe implemented in a wide range of air vehicles having one or moretiltable propulsion units. Such air vehicles may be, by way of example,a tiltrotor air vehicle, a tiltwing air vehicle, a tiltjet air vehicle,an air vehicle propelled by a thrust vector or other type of tiltablejet engine, and so forth. It is noted that the group of air vehicles forwhich the invention may be implemented is not restricted to the aboveexamples, and other types of air vehicles may also be implemented. It isespecially noted that on top of the one or more tiltable propulsionunits, the air vehicle may have one or more additional powerplants apartfrom the aforementioned tiltable propulsion units—such as a rotor forsubstantially vertical thrust; a propeller for substantially horizontalthrust, a jet engine, and so forth.

Each such tiltable propulsion unit may include, for example, any one ofa tiltable ducted fan unit, a tiltable propeller unit, a tiltableturbojet unit, a tiltable turbofan unit, a tiltable propfan unit, and soon. Each such tiltable ducted fan unit may comprise at least one fan,and additionally or alternatively has an absence of vanes forcontrolling the respective said variable thrust vector. It is noted thatthe air vehicle may further include one or more non-tiltable propulsionunits, fixedly (i.e., non-tiltably) mountable to the air vehicle toprovide a thrust having a fixed thrust vector with respect thereto. Sucha non-tiltable propulsion unit may include any one of a non-tiltableducted fan unit, a non-tiltable propeller unit, a non-tiltable turbojetunit, a non-tiltable turbofan unit, a non-tiltable propfan unit. Forexample, such a non-tiltable ducted fan unit may include at least onefan and additionally or alternatively may have an absence of movablevanes for controlling the respective said vector.

FIGS. 1A, 1B, 2A, and 2B illustrate isometric views of air vehicle 100in different flight modes that are different from each other in at leastthe tilt angle of the tiltable propulsion units 420. It is noted thatwhile the below disclosed systems, methods, and computer programproducts may be implemented on an air vehicle such as the one disclosedin the WO 2010/137016 reference (that is incorporated herein in itsentirety) and even exemplified in relation to such an air vehicle, theinvention is in no way limited to such an air vehicle, and may beimplemented on a wide range of air vehicles having at least one tiltablepropulsion unit.

The aerodynamics of air vehicles with tiltable propulsion units isdifferent from those of conventional fixed-wing airplanes and from thoseof helicopters, to give examples of two of the most widely implementedforms of aircraft.

In conventional fixed-wing airplanes, most of the lift is gained fromflow of air around airfoils of the airplane, and especially its wings.Most of the thrust in airplanes is generated by engines usuallyoperating in the generally longitudinal thrust vector direction alignedwith the longitudinal axis of a fuselage of the airplane. While someairplanes implement a directed or vectored jet thrust for producing lift(e.g. the Harrier Jump Jet), and while some airplanes incorporate thrustvector control that enables a limited manipulation of a direction oftheir main jet (e.g. the TVC nozzles of the Sukhoi Su-30 MKI jetairplane can be deflected ±15 degrees in the vertical plane), the mainsource of lift—especially during flight of the airplane—is neverthelessthe wing.

In helicopters, in comparison, most of the lift is supplied by one ormore engine driven rotors, which also supply the thrust. While some liftmay be gained from various surfaces of the helicopter, especially duringflight (as opposed to hover), this lift is usually substantially smallerthan the lift provided by the rotary wing. It should be noted that whilesome thrust is provided by the tail rotor in many helicopter designs,this thrust does not contribute to propelling the helicopter but israther used to counter torque generated by the main rotor in suchdesigns.

In comparison to the aforementioned examples of other types ofconventional aircraft, when one or more tiltable propulsion units 420 ofthe air vehicle 100 are directed in general vertical thrust vectordirection (e.g. as exemplified in FIG. 1B), lift is generated both bythe tiltable propulsion units 420 and by wing 320 (as long as airvehicle 100 is flying at non-zero airspeed). This combination of liftgenerated both by the wing 320 and by the tiltable propulsion units 420also occurs to a lesser extent when the tiltable propulsion units 420are tilted to a diagonal position intermediate between the generalvertical thrust vector direction and the general longitudinal thrustvector direction.

It will therefore be appreciated by a person who is skilled in the artthat flight schemes that were suitable for either solely fixed wing orsolely rotary wing aircrafts are not suitable for an air vehicle 100having a tiltable propulsion unit 420 when the latter is directed in thegeneral vertical thrust vector direction or intermediate directions.Attempting to maneuver air vehicle 100 when in such a constellation inmanners borrowed from either solely fixed wing or solely rotary wingaircrafts is therefore bound to failure.

FIGS. 1A and 2A are front/top/side isometric views of two embodiments ofair vehicle 100 for which the invention may be implemented, when thetiltable propulsion units 420 of each of the exemplary air vehicles 100are directed in a general vertical thrust position. FIGS. 1B and 2B arefront/top/side isometric views of the respective air vehicles 100 ofFIGS. 1A and 2A, when the respective tiltable propulsion units 420 aredirected in a general longitudinal thrust position.

It should be noted that some possible implementations of air vehicles100 are exemplified in Patent Cooperation Treaty (PCT) applicationWO2010/137016 which is, as aforementioned, incorporated herein in itsentirety. It is noted that numeral references used in the WO2010/137016reference are used herein in the same meaning. It is noted that whilethat while air vehicle 100 is illustrated as a tilt-rotor aircraft,other types of aircraft that implement tiltable propulsion units—such astilt-wing configurations, for example—may also be implemented as airvehicle 100.

It is further noted that while air vehicle 100 is illustrated as asubsonic UAV, in alternative variations of this embodiment the airvehicle may be different, e.g. manned and/or configured as a transonicand/or supersonic air vehicle.

According to an aspect of the invention there is provided a controlsystem 1200, suitably mounted with respect to the air vehicle 100, andconfigured to control operation of air vehicle 100. The controlling ofthe operation of the air vehicle 100 by control system 1200 includesissuing of controlling commands to controllers of aerodynamic subsystemsof the air vehicle. For example, a command to tilt a position ofailerons 345 may be issued to one or more aileron controllers 346 whichhydraulically modify the position of the ailerons 345 with respect tothe wing 320. Such controllers of aerodynamic subsystems of the airvehicle (e.g. controller 346) may be a part of control system 1200,though this is not necessarily so.

It is also noted that, in some implementations, more than one system mayissue commands for a single controller of an aerodynamic subsystem. Forexample, the ailerons may be controlled independently by control system1200 that is mounted on air vehicle 100, and by a remote system operatedwirelessly by a ground operator. The control by different systems may becarried out sequentially (e.g. a human takeover of the controlling mayprevent automated control), and may also be enabled and possibly carriedout concurrently.

Control system 1200 may be the control system which controls air vehicle100 during its entire sequence operation (e.g. take-off, flight,maneuvering, landing, shutting off), but this is not necessarily so. Inany case, control system 1200 is at least configured to control adeceleration process of the air vehicle 100 (e.g. as disclosed below),and may optionally also be configured to control other operationsthereof.

Control system 1200 may be fully automated and autonomous, but in someimplementations it may also react to commands issued by another one ormore systems and/or persons. For example, if over-ridden by a humanissued command, control system 1200 may stop its autonomous control ofthe air vehicle 100, which is then controlled by another system or bythe issuing person (either by mediation of control system 1200 orotherwise).

Control system 1200 includes one or more inputs 1210 for receivinginformation indicative of monitored airspeed of air vehicle 100, and ofmonitored altitude thereof. The monitored airspeed of air vehicle 100may be detected by one or more airspeed detectors 490 (e.g. implementedas Pitot tubes), while the altitude of the air vehicle 100 may bedetermined by an altimeter (not illustrated, may be implemented, forexample, as a pressure altimeter, a sonic altimeter, a radar altimeter,a Global Positioning System (GPS) based altimeter, and so forth). It isnoted that inputs of the control system 1200 may also be used forreceiving information indicative of additional parameters pertaining toflight and state of air vehicle 100.

Such parameters, on which the controlling by control system 1200 may bebased, may include for example any one or more of the followingparameters:

-   -   a. Groundspeed;    -   b. Pitch, yaw, and roll;    -   c. Linear acceleration (along one or more axes);    -   d. Angular acceleration;    -   e. Time;    -   f. Weight of the aircraft (of which center of mass may possibly        be deducted, e.g. if deviation from take-off weight results from        fuel consumption and/or dispatching of known weights);    -   g. Energy level (e.g. residual battery charge level, fuel        level);    -   h. State of one or more aerodynamic subsystems (e.g. position of        ailerons, etc.);    -   i. Environmental atmospheric conditions;

As will be described in more detail herein, the control system 1200 mayprovide the desired control for the air vehicle 100 in accordance withone or more particular targets or a goal. For example, control system1200 may control air vehicle 100 for landing it in a predeterminedlocation, for bringing it to a hover at a determined location, etc.Clearly, other such goals may also be implemented by control system1200—e.g. flying air vehicle 100 along a predetermined course, flying itto a predetermined location in an energetically efficient way,controlling its take-off, etc.

Reverting to the examples of landing air vehicle 100 or bringing it to ahover, some examples of additional parameters that may be used bycontrol system 1200 (which may be defined by control system 1200, byanother system, and/or by a person) are:

-   -   a. Where should the air vehicle land or hover? (e.g. what is the        hover destination position? What is the final landing        destination?)    -   b. At what height should the air vehicle hover?    -   c. From which direction should the air vehicle arrive? (e.g.        against the wind, at an azimuth of 271°, when a captured image        by a camera mounted on the air vehicle matches one or more        reference images, etc.)    -   d. At what access angle should the air vehicle descend?    -   e. What are the horizontal ranges allocated to some or all of        the different sub-stages?    -   f. What are the timing constraints for the landing?

Control system 1200 further includes at least one control unit 1220 thatis configured to issue controlling commands to controllers (e.g.controller 346) of aerodynamic subsystems (e.g. aileron 345, othercontrol surfaces, etc.) of air vehicle 100. The aerodynamic subsystemsincludes, among others, the at least one tiltable propulsion unit 420.

Control unit 1220 may be the control system which controls air vehicle100 during its entire sequence operation (e.g. take-off, flight,maneuvering, landing, shutting off), but this is not necessarily so.Control unit 1220 is configured to issue commands at least when controlsystem 1200 controls the deceleration process of the air vehicle 100(and at least for this goal), and may also be configured to issuecommands for controlling other operations thereof. Control unit 1220 maybe fully automated and autonomous, but in some implementations it mayalso react to commands issued by another one or more systems and/orpersons. For example, if over-ridden by a human issued command, controlunit 1220 may stop issuing commands autonomously, and air vehicle 100may then be controlled by another system or by the issuing person(either by mediation of control unit 1220 or otherwise).

Control unit 1220 is configured to issue the controlling commands atleast for:

(a) controlling, during a descent of air vehicle 100, a descendingcourse of air vehicle 100 based on at least monitored airspeed andmonitored altitude of the air vehicle 100, at least by: (i) controllingin a first part of the descent, an operation of the at least onetiltable propulsion unit 420 to provide thrust in the generallongitudinal thrust vector direction for propelling air vehicle 100; and(ii) following a tilting of the at least one tiltable propulsion unit420, controlling in a second part of the descent an operation of the atleast one tiltable propulsion unit 420 to provide thrust in the generalvertical thrust vector direction for providing lift to air vehicle 100;and

(b) controlling a reducing of the groundspeed of air vehicle 100substantially to a hover, while the at least one tiltable propulsionunit 420 provides thrust in the general vertical thrust vectordirection, at least by controlling thrust power of the at least onetiltable propulsion unit 420 for reducing a difference between measuredgroundspeed of the air vehicle 100 and set groundspeed, whilerestricting reduction of the thrust power based on a lower threshold1720 (FIG. 5) that is determined in response to a measured airspeed ofthe air vehicle 100.

Such commands issued by control unit 1220 may be commands issued tocontrollers of aerodynamic subsystems of air vehicle 100, but othertypes of commands may also be issued. For example, control unit 1220 mayalso issue a command (for the same goal) to dedicated processing units,to databases (for retrieval of data and for storing it for later use),to sensors for requesting data, to communication systems forcommunicating with off-board systems, and so forth. Control unit 1220may include one or more processors, and/or one or more dedicatedprocessing modules—each of which may be implemented in hardware,software, firmware, or any combination thereof.

The controlling of air vehicle 100 by control system 1200 as discussedabove involves balancing and compensating between many contradictoryrequirements. As will be discussed below, the aerodynamic regime of atilt rotor air vehicle is unique in many ways. For example, the stallingbehavior of such an air vehicle is substantially different from that ofa conventional fixed-wing aircraft but also from that of a helicopter orother rotary wing aircraft. A more detailed discussion is providedbelow, with respect to control system 1200 and also with respect tomethods 1500 and 1600.

While not necessarily so, control unit 1220 (and control system 1200 ingeneral) may control the descending course of air vehicle 100 bycarrying out method 1500. According to an embodiment of the invention,control unit 1220 may also be configured to implement method 1600.Therefore, a discussion of these two methods is presented below, and thediscussion of control system 1200 is continued thereafter. However,implementation of methods 1500 and 1600 is not restricted to controlsystems such as control system 1200.

FIG. 3A is a flow chart of method 1500, according to an embodiment ofthe invention. Method 1500 is a method for controlling a decelerationprocess of an air vehicle which includes at least one tiltablepropulsion unit, each of the at least one tiltable propulsion units istiltable to provide a thrust whose direction is variable at leastbetween a general vertical thrust vector direction and a generallongitudinal thrust vector direction with respect to the air vehicle.Referring to the examples set forth in the previous drawings, it isnoted that method 1500 may be implemented to control a decelerationprocess of an air vehicle such as air vehicle 100. However, this is notnecessarily so, and method 1500 may also be used to control decelerationprocesses of other types of air vehicles including one or more tiltablepropulsion units 420.

Referring to the examples set forth in the drawings, it is noted thatwhile not necessarily so, method 1500 may be carried out by a controlsystem such as control system 1200. In various implementations, method1500 may be carried out—wholly or partly, by an on-board system, by aremote system (e.g. ground system or airborne system), and/or by humanintervention, as well as by any combination thereof.

It is noted that the following description pertaining to method 1500(and the following descriptions pertaining to additional methods) isstructured in an orderly way. That is, if stage A is carried out beforestage B, alternative implementations and variations of stage A will bediscussed before any substantial discussion of stage B. The reader maytherefore benefit from reading the following description in view of therespective figures, in order to see what part each of the stages maytake in the overall flow of method 1500, in at least one embodimentthereof.

During the description of method 1500, reference will be made to FIGS.4A and 4B, which illustrate two possible example flight courses 1010 and1020 of the air vehicle in which the air vehicle decelerates.

Ultimately, the deceleration process of method 1500 may be used toreduce the groundspeed of the air vehicle substantially to a hoverand/or to provide a vertical landing. A destination location of therespective deceleration path may be determined in advance, but this isnot necessarily so. For example, the deceleration process may becontrolled to achieve hovering above a given location defined bygeographical coordinates or for landing in another such location, it maybe controlled for achieving landing or hovering at a predeterminedheight, within a given distance, or at other relative positioning, andit may also be controlled to land or hover within a given time frame.The destination may be updated from time to time (e.g. if required tohover or land above a target that is in motion when the destination isdetermined). A wide variety of additional exemplary scenarios willreadily be apparent to the skillful practitioner.

It should be noted that in various implementations of method 1500, thedecelerating process may address different needs, and may enableexecution of method 1500 (and/or of specific stages thereof) withinconstraints that are stricter than previously possible. For example,while prior art tiltrotor aircrafts are known to have decelerated andeven landed, doing so in a sufficiently quick a manner, within asufficiently small distance, and/or losing substantial height in theprocess, may turn out to be impractical using prior art schemes. Someexamples of scenarios are provided below.

It is noted that method 1500 includes several stages of controlling, asis disclosed below in detail. Such controlling may be implemented invarious ways. Such controlling may be implemented by a pilot, by anotherperson onboard, and by a remote human operator (e.g. for an unmannedtilt rotor air vehicle). However, method 1500 may also be implemented byone or more computerized systems (e.g. as exemplified in relation tosystem 1200). Such a system may be mounted onboard the air vehicle ofmethod 1500, or externally, and multiple such systems may coordinate toimplement method 1500 (wherein each stage of the method may beimplemented by a single system or a combination of such computerizedsystems). Additionally, a combination of one or more human controllersand one or more computerized systems may also be implemented.

According to an embodiment of the invention, the controlling of thedescending course and the controlling of the reducing of the groundspeed(both of which are disclosed in greater detail below) include automatedcontrolling by at least one processor of a control unit mounted on theair vehicle. It is noted that such processors and/or other computerizedsystems may be a dedicated system (implemented in hardware, firmware,etc.), and may also be implemented in software run by a processor ofanother system mounted on the air vehicle.

It is also noted that different stages of method 1500 includecontrolling (e.g. controlling a reducing of groundspeed of the airvehicle in stage 1550). While not necessarily so, in each of thecontrolling stages method 1500 may possibly also include the carryingout of the controlled operation, even if not explicitly elaborated so.Continuing the same example, on top of the controlling of stage 1550,method 1500 may further include reducing the groundspeed of the airvehicle substantially to a hover.

Method 1500 starts with stage 1510 that is carried out during a descentof the air vehicle. Referring also to the examples set forth in FIG. 4Aand FIG. 4B, stage 1510 may be carried out during part 1011 of course1010 (or part 1021 of course 1020), but this is not necessarily so. Itshould be noted that the flight course of the air vehicle during thedescent of stage 1510 is not necessarily a strictly monotonic descendingcourse, and that while an altitude of the air vehicle at the end of thedescent is substantially lower than its altitude at the beginning of thedescent, the air vehicle may nevertheless experience some temporaryascents (e.g. due to unexpected winds or air conditions, due to movingof control surfaces of the air vehicle, and even as effects of actionstaken as part of the controlling of stage 1510—e.g. in order to keep theair vehicle within an envelope that ultimately permits deceleration tosubstantial hover at a predetermined hover destination position). Thedescending course may include descent of the air vehicle in asubstantial fraction thereof—e.g. for 80% of its duration, and possiblyeven more (e.g. 90%, 95%, etc).

Stage 1510 that is carried out, as aforementioned, during a descent ofthe air vehicle includes controlling a descending course of the airvehicle based on at least monitored airspeed and monitored altitude ofthe air vehicle. Referring to the examples set forth in FIGS. 4A and 4B,stage 1510 may be carried out by a control unit such as control unit1220.

Controlling of the course of the air vehicle may be achieved at least bycontrolling an operation of one or more of the aerodynamic subsystems ofthe air vehicle. Such parts may include, by way of example, the at leastone tiltable propulsion unit, at least one non-tiltable propulsion unit,a throttle, an engine, ailerons, elevators, rudder, ruddervator,flaperons, elevons, wing flaps, slats, spoilers, air brakes,variable-sweep wings, non-tiltable propulsion unit, blades of rotors,and so on. It is noted that different stages of method 1500 (e.g. thecontrolling of stage 1510, 1520, 1530, 1540, 1550, or of any combinationthereof) may include controlling an operation of at least oneaerodynamic subsystem of the air vehicle selected from a groupconsisting of an aileron, an elevator, a rudder, a ruddervator, aflaperon, elevons, and a wing flap.

The controlling of such aerodynamic subsystems (and/or other parts) maybe achieved in various ways, such as by issuing instructions to suchparts, or to components controlling such parts. In a few exemplaryimplementations, instructions may be implemented by modifying anelectric current transmitted to servos controlling such parts, byinstructing a hydraulic pump to modify a pressure in a pipe leading tosuch a part, and so forth. In other examples, the controlling may beachieved by physical means. For example, if method 1500 is wholly orpartly carried out by a pilot (or other person onboard), that pilot maychange a physical state of one or more component—e.g. push a throttle.It is noted that physical means for controlling the course may also beimplemented by systems and not only by humans, as will be clear to aperson who is of skill in the art.

It should be noted that method 1500 may include not only the controllingof the operation of one or more of the aerodynamic subsystems of the airvehicle or other components/systems thereof, but also the actualoperation thereof. In an example, while the controlling may be carriedout by one or more people, processors, controllers, or like systems(different implementations and combinations thereof are possible), theoperating of the different parts/components/systems of the air vehiclemay be carried out by other parts/components/systems mounted on the airvehicle.

Referring to the descending course of the air vehicle, it should benoted that the controlling of the course of the air vehicle in stage1510 may include controlling temporal and/or spatial aspects of it. Forexample, the controlling may include controlling of some or all of thefollowing parameters—the speed of the air vehicle (or components thereofsuch as groundspeed, airspeed, descending speed, and so forth),controlling its arriving to predetermined location at a certain timing,controlling its altitude, its horizontal positioning, its pitch, itsturn, its yaw, its direction, and so on and so forth.

The controlling of the course may include controlling the course atleast for keeping the air vehicle within an envelope that ultimatelypermits its deceleration to substantially a hover at a predeterminedhover destination position, or which ultimately permits accomplishinganother goal. It is noted that such an envelope may not be the largestenvelope permitting such a deceleration (or reaching of such othergoal), but rather an envelope defined in view of such a goal. Some orall of the parameters defining such an envelope may also be definedregardless of the final destination, e.g. resulting from aerodynamicconsiderations (for example prevention of reaching a stalling angle,keeping direction against the wind), from tactical requirements (e.g.reducing an exposure period above/below given height), for requirementsof another system of the air vehicle or system carried by it (e.g. forpreventing damage to a sensitive camera payload), and so forth.

While the term envelope is a term widely used in the art and asaforementioned may carry meaning as understood by one of ordinary skillin the art, it is noted that this term may be regarded as including atleast one or more of the following sets of parameters: a set ofperformance limits (e.g. of the aircraft) that may not be safelyexceeded, a set of operating parameters that exists within these limits,and a set of spatial and/or temporal parameters relating to courseparameters.

Stage 1510 that is carried out, as aforementioned, during a descent ofthe air vehicle, includes controlling a descending course of the airvehicle based on at least monitored airspeed and monitored altitude ofthe air vehicle.

The controlling of the descending course of stage 1510 includes at leaststage 1520 that is carried out in a first part of the descent (e.g. part1012 of course 1010, and part 1022 of course 1020), and stage 1540 thatis carried out in a second part of the descent (e.g. part 1013 of course1010, and part 1023 of course 1020), wherein the second part of thecourse comes after the first part of the course. While the second partmay be a direct continuation of the first part of the course (e.g. asillustrated in FIG. 4A), this is not necessarily so, and the two partsmay be remote from each other—both geometrically and temporally (e.g. asexemplified in FIG. 4B). Regardless of whether the second part is adirect continuation of the first part of the course or not, stage 1510includes carrying out stages 1520 and 1540 in that order.

Stage 1520, that is carried out in the first part of the descent,includes controlling an operation of the at least one tiltablepropulsion unit to provide thrust in the general longitudinal thrustvector direction for propelling the air vehicle. Especially, thecontrolling of stage 1520 may include controlling the operation of theat least one tiltable propulsion unit to provide thrust when directed inthe general longitudinal thrust position (e.g. similar to thepositioning of the tiltable propulsion units 420 in FIG. 1A). It isnoted that controlling an operation of other components of the airvehicle may also be carried out during the first part of the descent.During this stage, the general longitudinal thrust vector direction (andthe longitudinal axis of the air vehicle) may be controllably inclinedto the horizontal at a nose-up angle, for enabling a descent of the airvehicle.

Optionally, the controlling of stage 1520 may include controlling theoperation of any of the one or more tiltable propulsion units mounted onthe air vehicle to provide thrust in the general longitudinal thrustvector direction (or at least, of any of the one or more tiltablepropulsion units mounted on the air vehicle that may be tilted toprovide thrust in the general longitudinal thrust vector direction, ifnot all tiltable propulsion units may be tilted to such a position).

The controlling of stage 1520 may include controlling the operation ofany active tiltable propulsion unit mounted on the air vehicle toprovide thrust in the general longitudinal thrust vector direction (e.g.if one or more of the tiltable propulsion units may be selectivelydeactivated).

The controlling of stage 1520 may include controlling the at least onetiltable propulsion unit for controlling the descending course of theair vehicle based on at least monitored airspeed and monitored altitudeof the air vehicle. It is noted that the controlling of the descendingcourse during the first part of the descent may include controlling thedescending course of the air vehicle at least by controlling anoperation of additional components of the air vehicle. Asaforementioned, controlling of the course of the air vehicle may beachieved also during the first part of the descent at least bycontrolling an operation of one or more of the aerodynamic subsystems ofthe air vehicle, e.g. as exemplified above. This controlling may includecontrolling various systems of the air vehicle, for keeping the airvehicle within an operational envelope whose parameters are controllablydefined for enabling the descent and/or slowing down of the air vehicle.

It is noted that in some implementations, a primary way of controllingthe descending course of the air vehicle (and especially its rate ofdescent) during the first part of the descent is by controlling thepitch and the speed of the air vehicle. The descending in suchimplementations may be primarily achieved at that stage of the descentby pitching a nose of the air vehicle down (e.g. below the horizon), sothat propelling of the air vehicle in the general longitudinal thrustvector direction has a vertical component directed downwards. In suchimplementations, a direction in which the air vehicle progresses mayprimarily be affected by the pitch of the air vehicle at that time. Itshould be noted that this is not necessarily so, and other parametersmay also significantly affect a direction of the air vehicle—e.g. ifgenerally vertical non-tiltable propulsion units are activated duringthe descent.

In a few examples, controlling a pitch of the air vehicle during thefirst part of the descent may include controlling an operation of atleast one elevator of the air vehicle (if implemented); controlling aroll of the air vehicle during the first part of the descent may includecontrolling an operation of at least one aileron of the air vehicle (ifimplemented) and possibly also of a rudder thereof (if implemented);controlling a yaw of the air vehicle during the first part of thedescent may include controlling an operation of at least one rudder ofthe air vehicle (if implemented) and possibly also of at least oneaileron thereof (if implemented). Controlling of aerodynamic componentsof the air vehicle for controlling the descent course thereof during atleast the first stage naturally depends on the type, shape; amount,size, etc. of such aerodynamic components implemented in any givenimplementation of the invention, and therefore goes beyond the scope ofthis disclosure.

The controlling of the descending course may further include controllingparameters that are not directly related for the descending. Also, someparameters which at first glance may seem of little relevance to thedescending (e.g. yaw), may also be controlled for effective control ofthe descending course. For example, controlling the yaw may be requiredfor keeping the air vehicle at a desired angle with respect to the windand/or for compensating for wind effects, while control of the roll mayalso compensate for wind conditions or other instabilities of the airvehicle.

Stage 1540, that is carried out in the second part of the descent,includes controlling an operation of the at least one tiltablepropulsion unit to provide thrust in the general vertical thrust vectordirection. The provisioning of thrust in the general vertical thrustvector direction may be controlled for providing lift to the airvehicle. Especially, the controlling of stage 1540 may includecontrolling the operation of the at least one tiltable propulsion unitto provide thrust when directed in the general vertical position (e.g.similar to the positioning of the tiltable propulsion units 420 in FIG.1B). Referring to the examples set forth in the previous drawings, stage1540 may be carried out by a control unit such as control unit 1220.

Conveniently, stage 1540 may be carried out following a tilting of theat least one tiltable propulsion unit to a position in which it providesthrust vector in the general vertical thrust direction. It should benoted that the general vertical thrust vector direction is notnecessarily perpendicular to the longitudinal axis of the aircraft, butis rather generally directed along the gravity operation direction. Forexample, the air vehicle may be inclined with respect to the horizon(e.g. it is in a nose-up or nose-down position), while the at least onetiltable propulsion unit is directed substantially in the direction ofgravitation.

Especially stage 1540 may be carried out following a tilting of the atleast one tiltable propulsion unit to a position in which the at leastone tiltable propulsion unit provides thrust vector whose component(projection) in the general vertical thrust direction is substantiallylarger than its component (projection) in the general longitudinalthrust vector direction (e.g. at least 2 times larger, at least 5 timeslarger, at least 20 times larger, and so on).

Stage 1540 may be carried out following a tilting to the generalvertical thrust vector direction of any active tiltable propulsion unitthat may possibly be tilted in that direction. It is noted that in anyof the above examples, stage 1540 may be preceded by controlling anoperation of one or more of the at least one tiltable propulsion unitsof the air vehicle following an at least partial tilting of thattiltable propulsion unit to a position in which at least part of itsthrust is provided in the general vertical thrust direction.

In the discussion of stage 1520, it is mentioned that the operation ofthe at least one tiltable propulsion unit is controlled to providethrust in the general longitudinal thrust vector direction forpropelling the air vehicle. It is noted that other propulsion units(especially non-tiltable ones) may also be used during that part of thecourse for providing thrust in the general longitudinal thrust vectordirection, but this is not necessarily so. Especially, in someimplementations, all the thrust provided to the air vehicle in thegeneral longitudinal thrust vector direction during the first part ofthe descent is provided by the at least one tiltable propulsion unit.

It is noted that an angle of the at least one tiltable propulsion unitwith respect to the air vehicle changes with speed in the first flightmode, e.g. by being lower than a fuselage main axis as the air vehiclegoes faster (an angle “lower” than the fuselage main axis means that asymmetry axis of the propulsion unit is directed more towards the bottomof the air vehicle rather than towards its top). This may be implementedand controlled in order, for example, to maintain angle of attack). Thecontrolling of the direction of the tiltable propulsion unit below thefuselage main axis may be limited, e.g. up to 10 degrees below thefuselage main axis.

Optionally, method 1500 may include stage 1530 that precedes stage 1540and which includes controlling a tilting of the at least one tiltablepropulsion unit, e.g. to a position in which it provides thrust vectorin the general vertical thrust direction. For example, the controllingof stage 1530 may include controlling the tilting of the at least onetiltable propulsion unit to a position in which the at least onetiltable propulsion unit provides thrust vector whose component(projection) in the general vertical thrust direction is substantiallylarger than its component (projection) in the general longitudinalthrust vector direction (e.g. at least 2 times larger, at least 5 timeslarger, at least 20 times larger, and so on). Referring to the examplesset forth in the previous drawings, stage 1530 may be carried out by acontrol unit such as control unit 1220.

It is noted that method 1500 may include a stage of tilting the at leastone tiltable propulsion unit, e.g. to a position in which it providesthrust vector in the general vertical thrust direction.

In different implementations, the tilting may be carried out indifferent ways, and according to different schemes. It is noted thatvarious such ways and schemes may be performed in a single systemimplementing method 1500, wherein in each case the actual scheme to beimplemented may be selected, for example, according to environmentalconditions, aerodynamic conditions, a state of the air vehicle, and soforth.

For example, the controlling of the tilting may include controlling atiming of the tilting (e.g. in respect to the fastest possible tiltingtime in a given system), a degree of the tilting, the number and orderof tiltable propulsion units tilted—and to what degree, tiltingdifferent units concurrently, partly concurrently, or sequentially, anoperation of a tiltable propulsion unit during its tilting (e.g. thrustprovided by it), and so forth.

In an example, stage 1530 may include stage 1532 of controlling areducing of a rotation speed of at least one rotating component of oneor more of the at least one tiltable propulsion units (e.g. a fan, arotor, and/or an engine thereof) prior to a completion of the tilting ofthat one or more units in stage 1530 (and possibly prior to acommencement of such tilting). The control of the reducing of therotation speed in stage 1532 may include controlling a breaking of arotating component of a respective tiltable propulsion unit.

In an example, when making the transition from a first flight mode (inwhich tiltable propulsion units of the air vehicle are directed toprovide thrust in the general longitudinal thrust vector direction to asecond flight mode in which the tiltable propulsion units are directedin the general vertical direction (e.g. when wishing to land or hoverafter being in forward flight), motor rotors of the respective at leastone tiltable propulsion unit of the air vehicle may be subjected to abraking procedure. Following such tilting of the at least one tiltablepropulsion unit (or after most of such tilting was carried out), therespective motors may be activated (e.g. immediately) and run up to therequired speed to provide the required thrust in the generallongitudinal thrust vector direction. In implementations of theinvention in which such breaking and/or reduction is carried out, aforward momentum of the air vehicle prior to the transition shouldsuffice to ensure that enough aerodynamic lift is generated (e.g. due toflowing of air around a wing of the air vehicle) for supporting the airvehicle during transition.

A feature of the procedure of braking and/or reduction of a thrustprovided by one or more of the at least one tiltable propulsion units(if implemented) is that it minimizes or reduces to zero angularmomentum (e.g. of rotors of the tiltable propulsion unit) on the atleast one tiltable propulsion unit during this transition relative towhat the such gyroscope-like effects would be if the rotating parts ofthe tiltable propulsion unit were still spinning at or close to theiroriginal angular velocity. Therefore, such a reduction or braking, ifimplemented, reduces the energy required for performing the transitionas well as the elapsed time thereof, and also enables utilization ofless massive and durable parts (e.g. in hinges and/or other componentsused for connecting the respective tiltable propulsion unit to afuselage of the air vehicle) and/or of parts of lesser capabilities(e.g. drive mechanism for tilting the tiltable propulsion units can beless powerful, and typically smaller and lighter, than would otherwisebe the case).

Air vehicle 100 may be configured for providing (e.g. by control unit1220) a braking procedure to each tiltable propulsion unit 420, therebyenabling the motor 421 of each tiltable propulsion unit 420 to bestopped or at least considerably slowed down, in a relatively short timeperiod, for example less than 1 second, thus destroying or significantlyreducing the angular momentum in tiltable propulsion unit 420, e.g. ofthe motor rotor and fan 426.

According to an embodiment of the invention, at most the ensuing reducedangular momentum after the braking procedure is a very small proportionof the angular momentum of the respective rotating part of the tiltablepropulsion unit during the first mode flight mode just preceding thebraking procedure—such a proportion may be 50%, preferably 40%, morepreferably 30%, more preferably 20%, more preferably 10%, morepreferably 5%, more preferably 1%, more preferably less than 1%.

It is noted that such breaking or reduction of rotation speed is notcompulsory, and possibly all rotating components of the at least onetiltable propulsion unit can each be rotated between the vertical thrustand horizontal thrust positions while continuously providing thrust, therespective thrust vector changing between zero and 90 degrees. Possibly,such a breaking or reduction procedure may be selectively implementedupon need.

Referring again to stage 1530 of controlling a tilting, it is noted thatin different implementations, the at least one tiltable propulsion unitmay be controllably tilted in stage 1530 to different extents. Accordingto various implementations, the at least one tiltable propulsion unitcan be rotated about any set tilt angle, ranging from (and including)zero to 90 degrees, or less than zero, or beyond 90 degrees, as desired,and remain at these respective tilt angles.

Method 1500 may further include optional stage 1534 of determiningtiming for tilting of the at least one tiltable propulsion unit betweenthe first and the second parts of the descent for minimizing a durationbetween the tilting and the substantial hover.

While not necessarily so, operating the air vehicle (and especially anytiltable propulsion units thereof) may be substantially more energyconsuming when in the second flight mode (in which the tiltablepropulsion units are directed in the general vertical direction) thanwhen in the first flight mode (in which tiltable propulsion units of theair vehicle are directed to provide thrust in the general horizontaldirection). In an example implementation of a 70 Kg tiltrotor UAV, oneminute of operation in the second flight mode may consume energyequivalent of some 10 to 20 minutes of flight in the first flight mode.

While not necessarily so, some performance parameters may also havelesser values in the second flight mode than their equivalents in thefirst flight mode. In an example, a maximum speed may be less, amaneuverability of the air vehicle may be less, and so forth.

Therefore in at least such scenarios, saving of energy (as well as otherpotential preferred conditions) may be achieved by minimizing theduration between the tilting and the substantial hover. It is howevernoted that the determining of the timing for the tilting may be based onother parameters apart from the duration between the tiling and theexpected hover, which may restrict the minimizing of that duration. Forexample, desirable margins of error may be kept for safetyconsiderations, maneuverability considerations in different parts of theexpected course may also be taken into account, and so forth.

Stage 1534 may include determining timing for tilting of the at leastone tiltable propulsion unit between the first and the second parts ofthe descent for minimizing a duration between the tilting of the atleast one tiltable propulsion unit and the substantial hover. Thedetermining of the timing in stage 1534 may be based on a range ofpossible aerodynamic states during the aforementioned duration betweenthe tilting and the expected substantial hover.

Clearly, if stage 1534 is carried out, the controlling of the tilting instage 1530 may be responsive to the determined timing. Method 1500 mayinclude a stage of providing the timing determined in stage 1534 to atleast one unit that participates in the controlling of the tiling of theat least one tiltable propulsion unit.

Returning to stage 1540 that is carried out in the second part of thedescent, and which includes controlling an operation of the at least onetiltable propulsion unit to provide thrust in the general verticalthrust vector direction. It is noted that method 1500 may furtherinclude a stage (denoted 1542) of controlling the operation of the atleast one tiltable propulsion unit to provide thrust in the generallongitudinal thrust vector direction during the second part of thedescent—in addition to the provisioning of thrust in the generalvertical thrust vector direction by the same at least one tiltablepropulsion unit. Stage 1542 may be carried out at least partlyconcurrently with the controlling of the operation of the at least onetiltable propulsion unit to provide thrust in the general verticalthrust vector direction.

Provisioning of thrust in the general longitudinal thrust vectordirection may be carried out for providing thrust for propulsion. It isnoted that controlling an operation of a propulsion unit to providethrust in both the general vertical thrust vector direction and thegeneral longitudinal thrust vector direction may be achieved in variousways. In but a few examples, a tiltable propulsion unit (e.g. a tiltablejet) may be tilted to an off-vertical angle in relation to a fuselage ofthe air vehicle, the vanes of a ducted fan may be moved, the pitch ofrotor blades of a rotor may be changed cyclically for tilting a rotordisk in a particular direction (e.g. similarly to some helicopters), andso on.

Method 1500 may also include determining a trade-off betweenprovisioning of thrust in the general vertical thrust vector directionand in the general longitudinal thrust vector direction, during thesecond part of the descent—when the at least one tiltable propulsionunit is directed in the general vertical direction (e.g. within 10° ofit).

It is noted that in implementations in which more than one tiltablepropulsion unit is implemented, the controlling of stage 1540 mayinclude controlling of one, some, or all of the tiltable propulsionunits of the air vehicle in the second part of the descent. Possibly,the controlling of stage 1540 includes controlling the operation of anyof the one or more tiltable propulsion units mounted on the air vehicleto provide thrust in the general vertical thrust vector direction (or atleast, of any of the one or more tiltable propulsion units mounted onthe air vehicle that may be tilted to provide thrust in the generalvertical thrust vector direction, if not all tiltable propulsion unitsmay be tilted to such a position). The controlling of stage 1540 mayinclude controlling the operation of any active tiltable propulsion unitmounted on the air vehicle to provide thrust in the general verticalthrust vector direction (e.g. if one or more of the tiltable propulsionunits may be selectively deactivated).

It is noted that controlling an operation of other components of the airvehicle may also be carried out during the second part of the descent,e.g. for controlling some or all of the following parameters—the speedof the air vehicle (or components thereof such as groundspeed, airspeed,descending speed, and so forth), its altitude, its horizontalpositioning, its pitch, its turn, its yaw, its direction, and so on.

The controlling of stage 1540 may include controlling the at least onetiltable propulsion unit for controlling the descending course of theair vehicle based on at least monitored airspeed and monitored altitudeof the air vehicle. It is noted that the controlling of the descendingcourse during the first part of the descent may include controlling thedescending course of the air vehicle by controlling an operation ofadditional components of the air vehicle. As aforementioned, controllingof the course of the air vehicle may be achieved also during the secondpart of the descent at least by controlling an operation of one or moreof the aerodynamic subsystems of the air vehicle, e.g. as exemplifiedabove.

By way of example, controlling of pitch and roll may be carried out bycontrolling a changing of a direction of a thrust generating componentof the respective tiltable propulsion unit (either by tilting thetiltable propulsion unit and/or by changing a direction of suchcomponent with respect to the tiltable propulsion unit itself),controlling of yaw may be carried out by controlling a tilting of adirection of thrust generated by two or more propulsion units indifferent directions (e.g. for providing opposing (or otherwise ofdifferent directions) horizontal thrust vectors to provide yawingmoments and/or side slip movements), controlling of altitude may becarried out at least by controlling power provided to a thrust providingcomponent of a respective tiltable propulsion unit or by modifying aconfiguration thereof (e.g. by changing pitch of its blades, if soimplemented), controlling of groundspeed may include controlling of adirection of a thrust of one or more of the tiltable propulsion units,or a thrust of a non-tiltable propulsion units providing thrust having acomponent in the general horizontal vector direction with respect to afuselage of the air vehicle, and so forth.

It is noted that method 1500 may further include stage 1544 carried outduring at least part of the second part of the descent, that includescontrolling an operating of at least one non-tiltable propulsion unit(if implemented) to provide thrust in the general vertical thrust vectordirection. Referring to the examples set forth in the previous drawings,stage 1544 may be carried out by a control unit such as control unit1220, and/or may include controlling the operating of a non-tiltablepropulsion unit (such as non-tiltable ducted fan 420 c of FIGS. 1A and1B).

It should be noted that even while the at least one tiltable propulsionunit is directed to provide thrust in the general vertical thrust vectordirection—and that possibly non-tiltable propulsion units (ifimplemented) of the air vehicle also provide thrust in the generalvertical thrust vector direction, the air vehicle still includescomponents that serve for controlling of the course of aircraft when theat least one tiltable propulsion unit was directed to provide thrust inthe general longitudinal thrust vector direction, and those componentsmay also be used for controlling of the air vehicle (e.g. its pitch,yaw, roll, and speed) also at this stage. Some such components that maybe implemented in various implementations are elevators, rudder,ailerons, etc.

Such components may be used for controlling the descending course of theair vehicle in stage 1540 (and possibly of a state of the air vehicleduring that course) also during the second part of the descent. In but afew examples, controlling a pitch of the air vehicle during the secondpart of the descent may possibly include controlling an operation of atleast one elevator of the air vehicle (if implemented); controlling aroll of the air vehicle during the second part of the descent mayinclude controlling an operation of at least one aileron of the airvehicle (if implemented) and possibly also of a rudder thereof (ifimplemented); controlling a yaw of the air vehicle during the secondpart of the descent may include controlling an operation of at least onerudder of the air vehicle (if implemented) and possibly also of at leastone aileron thereof (if implemented). Controlling of aerodynamiccomponents of the air vehicle for controlling the descent course thereofduring at least the second stage naturally depends on the type, shape;amount, size, etc. of such aerodynamic components implemented in anygiven implementation of the invention, and therefore goes beyond thescope of this disclosure.

Significantly, during the second part of the descent, the air vehiclestill includes a wing which provides substantial lift when airspeed ofthe air vehicle is still relatively high (as it is during at least thegreater part of the second part). Therefore, behavior (andconsequentially control) of the air vehicle during the second part isdifferent than that of other substantially vertical-thrust aircrafttypes (e.g. helicopter). For example, while in standard rotary wingaircraft reducing of thrust power below a given threshold would resultin plain descent due to lack of sufficient lift to maintain altitude, inthe air vehicle of method 1500 the wing would provide substantial liftand would make it difficult to reduce altitude quickly while theairspeed of the air vehicle is still high.

In a related aspect, in high enough airspeed of the air vehicle,substantial portion of the lift of the air vehicle is still provided bythe wing. Therefore, pitching backwards (e.g. as is sometimes done infixed wing airplanes as well as in prior art standard rotary wingaircraft types for slowing down) may result in crossing of the stallangle, sudden loss of lift from the wing, and stalling of the airvehicle.

It is noted that the stall speed of the air vehicle when in the secondflight mode (i.e. when the at least one tiltable propulsion unit isdirected in the substantial vertical direction) is substantially lowerthan the stall speed of that air vehicle when in the first flight mode(i.e. when the at least one tiltable propulsion unit is directed in thesubstantial horizontal direction), because of the additional liftprovided by the generally vertically directed tiltable propulsion unit(i.e. due to that lift, less additional lift is required from the wingto keep a certain vertical velocity, which can be gained at a relativelylower speed). This may cause significant aerodynamic difficulties torecover from a spin or other induced aerodynamically-problematic statesthat may result from stalling, e.g. when compared to stalling when inthe first flight mode of the air vehicle.

It is noted that in some implementations, a primary way of controllingthe descending course of the air vehicle (and especially its rate ofdescent) during its descent is by controlling the vertical thrustprovided by its propulsion units, and especially by the at least onetiltable propulsion unit. However, apart from the controllable liftdirectly controllable by the controlling of the at least one tiltablepropulsion unit, substantial lift also generated by other components ofthe air vehicle, and primarily by its one or more wings. Since a primarygoal of descending in such implementations may therefore requirereduction of lift of the air vehicle gained by the one or more wingsthereof, a speed of the air vehicle should be reduced in order to reducesuch lift in at least some implementations.

It is noted that reduction of lift may also be achieved by pitching down(thus reducing an attack angle of each of the one or more wings).However, this may result in the speeding up of air vehicle due togravity and possibly also to a horizontal thrust generated due to thetilting angle of the at least one tiltable propulsion unit. Descendingusually requires losing energy (at least gravitational potentialenergy). Since in addition, stage 1550 of method 1500 (that is carriedout at least partly after an initiation of the carrying out of stage1540) includes controlling a reducing of a groundspeed of the airvehicle substantially to a hover, there is usually a further incentiveto at least maintain (if not reduce) horizontal speed of the air vehicleduring stage 1540. Other means that may be controllably utilized forreducing of speed are increasing lift (e.g. deploying flaps), increasingangle of attack (but not stalling), etc.).

Therefore, in some implementations, a primary way of controlling thedescending course of the air vehicle (and especially its rate ofdescent) during its descent is by controlling a nose-up pitching of theair vehicle (when a nose of the air vehicle is pitched above horizontallevel) and by controlling a speed of the air vehicle.

A combined controlling of a nose-up pitching of the air vehicle and ofreducing horizontal forward thrust components (and even increasinghorizontal backward thrust component) results in mutually assistingprocesses.

On the one hand, nose-up pitching the air vehicle enables reduction ofthe horizontal forward speed—by generating drag on the wing and byassisting in tilting the at least one tiltable propulsion unit backwards(thus possibly creating a backward horizontal thrust component).

On the other hand, slowing the air vehicle down enables nose-up pitchingof the air vehicle (wherein such nose-up pitching would otherwisegenerate additional lift on the wing and possibly would have resulted inunset ascending of the air vehicle).

The thrust generated by the at least one tiltable propulsion unit, ifindeed tilted backwards due to the pitching backwards, serves opposingfactors in such a situation. On the one hand—a powerful thrust resultsin backward power that assists in slowing a horizontal speed of the airvehicle down (which as aforementioned may in turn assist in controllinga descent of the air vehicle). On the other hand, a powerful thrust bythe generally vertically directed at least one tiltable propulsion unitresults in additional lift that may disrupt an effort to lower analtitude of the air vehicle during its descent.

It would therefore become clear that controlling of the operation of theat least one tiltable propulsion unit during the descent is not an easytask—e.g. due to the opposing roles it serves and due to the hazardousresults that may yield if controlled erroneously—even when assisted bycontrolling of other components of the air vehicle, e.g. as describedabove. Additional implementations and considerations of the controllingof the at least one tiltable propulsion unit and/or additionalcomponents of the air vehicle during the descent will be discussed infurther detail below.

The controlling of the deceleration process may therefore includecontrolling the deceleration process of the air vehicle that includes awing (referring to the examples set forth in the previous drawings, thewing may be wing 320), and balancing between contradictory aerodynamiceffects resulting from the wing and from the at least one tiltablepropulsion unit.

The descending course—or at least one or more portions thereof—may becharacterized by an angle (or a slope) of the course with respect to thehorizon. Such angle characterizes the vertical distance the air planetravels for each unit of horizontal distance. For example, if the airvehicle loses in a part of the course about 14 cm in altitude for every100 cm horizontal distance traveled, then the corresponding slope is14%, and the corresponding angle is about 8°.

An access angle (or a range of access angles) may be determined and usedin later parts of the course traveled. The determining of such an accessangle (or corresponding range of angles) may be based on variousconsiderations—such as geometric considerations (e.g. distance from thedestination location and current altitude), atmospheric conditions (e.g.wind), aerodynamic and/or energetic efficiency, operationalconsiderations, capabilities of the air vehicle, and so forth.

Optionally, the controlling in the second part of the descent may bepreceded by determining a permitted air vehicle descending angle (or arange thereof—e.g. by determining two threshold angles) for at least apart of the descending course based on atmospheric conditions (or otherconditions e.g. as exemplified above), wherein the controlling of thedescending course during at least a part of the first part of thedescent is based on the permitted air vehicle descending angle. That is,in the discussed implementation the permitted air vehicle descendingangle (or angles) is determined either during the first part of thedescent (e.g. during part 1011 of the course) or even prior to theinitiation of the descent (e.g. upon receiving a command to land ormaking such a decision, or even receiving that angle from a remotesystem).

A permitted air vehicle descending angle (or a range thereof)—which maybe the angle determined for the first part of the descent but notnecessarily so—may also be determined for at least a part of the secondpart of the descending course (e.g. based on atmospheric conditions orother conditions), wherein the controlling of the descending courseduring at least a part of the second part of the descent is based onthat permitted air vehicle descending angle.

The controlling of either part of the descent course may include keepingthe actual descent rate of the air vehicle within the limits based onthe one or more permitted air vehicle descending angles. As can be seenin FIG. 7, for example, the actual descent angle of the course is keptduring both the first part of the descent and part of its second part,but the permitted air vehicle descending angle ranges is modifiedbetween the first and the second parts. It is noted that the permittedair vehicle descending angle may be revisited and determined many times(and even substantially continuously) during the descending of the airvehicle.

Method 1500 further includes stage 1550 of controlling the reducing of agroundspeed of the air vehicle substantially to a hover, while the atleast one tiltable propulsion unit provides thrust in the generalvertical thrust vector direction. Referring to the examples set forth inthe previous drawings, stage 1550 may be carried out by a control unitsuch as control unit 1220. Referring to the examples set forth in theprevious drawings, stage 1550 may be carried out during part 1014 ofcourse 1010 (or part 1024 of course 1020), but this is not necessarilyso. It is noted that stage 1550 may at least partly overlap stage 1540(and that controlling of reducing of groundspeed may also be carried outprior to stage 1540—e.g. when the at least one tiltable propulsion unitis directed in the substantially longitudinal direction). It istherefore to be understood that the part of the course in whichreduction of the groundspeed is carried out (and controlled)—e.g. part1024 of the course 1020—may at least partly overlap a part of the coursein which descent of the air vehicle is carried out and controlled—e.g.part 1021 (and especially part 1023) of course 1020.

It should be noted that the groundspeed of the air vehicle during thereduction of the groundspeed of stage 1550 is not necessarily a strictlymonotonic slowing down, and that while the groundspeed of the airvehicle at an end of the groundspeed reduction is substantially lowerthan its groundspeed at the beginning of the groundspeed reduction, theair vehicle may nevertheless experience some temporary acceleration(e.g. due to unexpected winds or air conditions, or moving of controlsurfaces of the air vehicle, and even as effects of actions taken aspart of the controlling of stage 1550—e.g. in order to keep the airvehicle within an envelope that ultimately permits deceleration to hoverat a predetermined hover destination position).

It should be noted that the flight course of the air vehicle duringstage 1550 is not necessarily a strictly monotonic decelerating course.While an altitude of the air vehicle at an end of stage 1550 may belower than its altitude at the beginning of that stage, this is notnecessarily so, and in other scenarios the air vehicle may even ascendduring at least some parts of stage 1550 (e.g. just before a finalslowing down, for example in order to quickly convert the kinetic energyof the flight to gravitational potential energy, thus quicklydiminishing the airspeed of the air vehicle, exemplified as 1016 in FIG.4A). Such an ascent as in 1016 may be implemented due to a possibledifficulty to diminish the final residual airspeed—e.g. the last 5 kts.An intended deliberate climbing may help in diminishing this airspeed,and may include, for example, a 5-10 meters ascent, and/or ascending in2.5 degrees above the horizon. It is noted that during this deliberateclimbing a deliberate stalling of the wings may be implemented.

Stage 1550 may conveniently include controlling reducing the groundspeedof the air vehicle based on at least monitored airspeed and monitoredgroundspeed of the air vehicle. Other monitored parameters may alsoserve as a basis to the controlling—e.g. monitored altitude of the airvehicle.

It is noted that while in a perfect theoretical hover the groundspeed ofan aircraft is exactly zero, reaching that theoretical standard this isnot practically achievable in practical systems, especially when subjectto varying environmental conditions. This is all the more problematic inimplementations in which the at least one tiltable propulsion unit, whendirected in the general vertical direction, may generate gusty air inthe vicinity of the air vehicle during an attempted hover of the latter,which acts against the fuselage and flight control surfaces.

Since in many cases a theoretical zero groundspeed hover is notpractically achievable for any significant time in at least someimplementations, it will be clear to a person who is skilled in the artthat in the substantial hover state some groundspeed and some deviationfrom a set location are permitted. The sizes of such error margins aredifferent in different implementations. For example, the air vehicle maybe permitted to hover in deviation of up to 1 m from the set hoverlocation, and in groundspeeds that do not exceed 2 m/sec, but othervalues are just as readily implementable. It is especially noted thatthe sizes of the error margins in the various possible implementationsmay be interrelated with the size of the air vehicle in suchimplementations, and with other characteristics thereof.

Controlling of the reduction of groundspeed of the air vehicle may beachieved at least by controlling an operation of one or more of theaerodynamic subsystems of the air vehicle. Such parts includes the atleast one tiltable propulsion unit and may also include by way ofexample, as aforementioned, at least one non-tiltable propulsion unit, athrottle, an engine, ailerons, elevators, rudder, ruddervator,flaperons, elevons, wing flaps, slats, spoilers, air brakes,variable-sweep wings, non-tiltable propulsion unit, blades of rotors,and so on. The controlling of such aerodynamic subsystems (and/or otherparts) may be achieved in various ways, such as those exemplified above,e.g. with relation to stage 1550. During reduction of the groundspeed,method 1500 may include controlling temporal and/or spatial aspects ofthe flight of the air vehicle. For example, the controlling may includecontrolling of some or all of the following parameters—the speed of theair vehicle (or components thereof such as groundspeed, airspeed,descending speed, and so forth), controlling its arriving to apredetermined location at a certain timing, controlling its altitude,its horizontal positioning, its pitch, its turn, its yaw, its direction,and so on and so forth.

According to an embodiment of the invention, the controlling of thereducing of the ground speed may include managing (or controlling) dragcreated by various components of the air vehicle. The managing of thedrag may include controlling operation of ailerons, potentiallyimplemented air brakes, and possibly also wheels. This management orcontrol of drag may be implemented autonomously, continuously, and mayinclude dynamic drag management.

While not necessarily so, the controlling of reduction of thegroundspeed may include and/or be at least partly concurrently carriedout with controlling a course of the air vehicle at least for keepingthe air vehicle within an envelope that ultimately permits decelerationto hover at a predetermined hover destination position, or whichultimately permits reaching another goal. It is noted that such anenvelope may not be the largest envelope permitting such a deceleration(or reaching of such other goal), but rather an envelope defined in viewof such a goal. Some or all of the parameters defining such an envelopemay also be defined regardless of the final destination, e.g. resultingfrom aerodynamic considerations (for example prevention of reaching astalling angle, keeping direction against the wind), from tacticalrequirements (e.g. reducing an exposure period above/below givenheight), for requirements of another system of the air vehicle or systemcarried by it (e.g. for preventing damage to a sensitive camerapayload), and so forth.

The controlling of the reducing of the groundspeed of the air vehiclesubstantially to a hover while the at least one tiltable propulsion unitprovides thrust in the general vertical thrust vector direction of stage1550 may possibly be implemented at least by carrying out stage 1560that includes controlling thrust power of the at least one tiltablepropulsion unit for reducing a difference between measured (or otherwiseestimated) groundspeed of the air vehicle and set groundspeed, whilerestricting reduction of the thrust power based on a lower thresholdthat is determined in response to a measured airspeed of the airvehicle. Referring to the examples set forth in the previous drawings,stage 1560 may be carried out by a control unit such as control unit1220.

It should be noted while the groundspeed is a monitored parameter whosereduction is controlled (e.g. in a controlled gradual process), theairspeed of the air vehicle cannot be ignored either, at least due toits direct impact on the lift generated on the wing portions. That is,while controlling components of the air vehicle for gradually reducingthe groundspeed (e.g. in order to reach a final destination stationarywith respect to the ground), the controlling is nevertheless carried outin response to the airspeed of the air vehicle, as the airspeed ratherthan the groundspeed is the speed that affects the aerodynamic behaviorof the air vehicle.

Referring to the term airspeed that is used throughout this disclosure,it is noted that airspeed pertains to a speed of the air vehiclerelative to the airmass in which it is flying. It is noted that indifferent implementations, and possibly also depending on context of thediscussion, different conventions may be used for the airspeed, amongwhich are:

-   -   Indicated airspeed, “IAS”, which is a reading of an airspeed        indicator (ASI) before any corrections are applied to it, (e.g.        for instrument, position, and other errors);    -   Calibrated airspeed, “CAS”, which is computed by applying        corrections for the indicated airspeed, e.g. for instrument        errors, position error (due to incorrect pressure at the static        port) and installation errors;    -   True airspeed, “TAS”, which is the real-world speed of the        aircraft relative to the airmass in which it is flying, and        which the calibrated airspeed is an attempted estimation of;    -   Equivalent airspeed, “EAS”, which is the airspeed at sea level        in the International Standard Atmosphere that would produce the        same dynamic pressure as the true airspeed (TAS) at the altitude        at which the air vehicle is flying, and which is the speed which        would be shown by an airspeed indicator with zero error in        low-speed flight;    -   Density airspeed, which is calibrated airspeed corrected for        pressure altitude and true air temperature.

It is noted that as various measures of airspeed are interdependent, aprocess may be responsive to more than one of such measures. Forexample, since calibrated airspeed is computed from the indicatedairspeed (which in turn depends on the true airspeed), a process that isbased on the calibrated airspeed is also based—albeit possiblyindirectly—on the indicated airspeed and/or on the true airspeed of theair vehicle.

For example, measurement and/or indication of airspeed may beaccomplished by an airspeed indicator (“ASI”) connected to apitot-static system, both of which are installed on the air vehicle. Thepitot-static system may include one or more pitot probes (or tubes)facing the on-coming air flow to measure pitot pressure, and one or morestatic ports to measure the static pressure in the air flow. These twopressures are compared by the ASI to give an IAS reading.

Referring to the term groundspeed that is used throughout thisdisclosure, it is noted that groundspeed pertains to the speed of an airvehicle relative to the ground above which the air vehicle is in flight,or to another reference coordinate system which is neither dependent onthe air vehicle, nor on conditions in its environment (e.g. acoordination system utilized by a global navigation satellite system(GNSS) such as a Global Positioning System (GPS)). It is noted that indifferent implementations, and possibly also depending on context of thediscussion, different measures may be used for the groundspeed, amongwhich are:

-   -   Indicated groundspeed, which is a reading of a groundspeed        indicating system;    -   Calibrated groundspeed, which is computed by applying        corrections for the indicated groundspeed; and    -   True groundspeed, which is the real-world speed of the air        vehicle relative to the ground or the other aforementioned        independent coordinate system, and which the calibrated        groundspeed is an attempted estimation of.

Determination of the groundspeed may be implemented in various ways. Forexample, groundspeed may be estimated by an inertial navigation system,by an external positioning system (e.g. GNSS), by vector-subtracting acurrent wind speed vector from a vector of the true airspeed of the airvehicle, navigation using landmarks, radio aided position location, andso forth. Clearly, more than one way of estimating groundspeed of theair vehicle may be utilized in some implementations of the invention.

The controlling of the reducing of the groundspeed of the air vehiclesubstantially to a hover of stage 1550 is therefore based not only onthe groundspeed of the air vehicle, but also on its airspeed. As will beclear to a person who is of skill in the art, the controlling of stage1550 may also be based on different combinations of various one or moreadditional parameters. Some of such additional parameters includeadditional speed parameters (e.g. descending speed), time, altitude,horizontal positioning, pitch, turn, yaw, direction of the air vehicle(e.g. absolute, with regard to the destination location, and/or withregard to a direction of the wind), and so on and so forth.

It is also noted that the controlling of stage 1550 may includecontrolling of some or all of the following parameters—the speed of theair vehicle (or components thereof such as groundspeed, airspeed,descending speed, and so forth), controlling its arriving at apredetermined location at a certain timing, controlling its altitude,its horizontal positioning, its pitch, its turn, its yaw, its direction,and so on and so forth.

Referring to stage 1560 that includes controlling thrust power of the atleast one tiltable propulsion unit for reducing a difference betweenmeasured groundspeed of the air vehicle and set groundspeed, whilerestricting reduction of the thrust power based on a lower thresholdthat is determined in response to a measured airspeed of the airvehicle, it is noted that the set groundspeed changes throughout thecourse of the air vehicle during stage 1550.

Since an expected state of the air vehicle is substantial hover, andsince stage 1560 starts before the air vehicle arrives at anypredetermined hover destination position, clearly the set groundspeedchanges over time from a forward flight groundspeed to substantiallyzero. However, it should be noted that the set groundspeed does notnecessarily continuously and/or monotonically lessen with time, and insome instances it may even increase for some intermediate durations. Forexample, changing winds may result in increasing of the set groundspeed(e.g. if an opposing wind strengthens, it may be desirable to reach avicinity of the destination hover location faster, because it isassessed that more drastic slowing down will be possible in a finalstage of the control reduction of groundspeed).

It is noted that while the determined groundspeed may be determineddirectly, it may also be determined as a derivate of a previouslydetermined entity, such as a planned course (which may also be modifiedfrom time to time).

Method 1500 may further include stage 1562 of determining the setgroundspeed, wherein the determining of the set groundspeed includesdetermining different speeds at different times—either in asubstantially continuous modification of the set groundspeed over timeor in discrete changes.

The determining of the set groundspeed may be carried out in response tothe distance of the air vehicle from a predetermined hover destinationposition. The actual determining may be carried out in differentdistances, but in other implementations a set groundspeed profile may begenerated, indicating the set groundspeeds for different such distances.It is also noted that the set groundspeed may be determined as a rangeof permitted groundspeeds, which may be determined based on the distancefrom the predetermined hover destination position.

It is noted that while stage 1562 is illustrated as part of stage 1560,it may be carried out independently and also at a different timing, andin fact may be a part of an optional stage 1590 of method 1500, thatincludes determining values of one or more spatial parameters (e.g.location, velocity) for the air vehicle.

Stage 1590 may include determining values for course setting parametersfor the air vehicle, but this is not necessarily so. Possibly, at leastsome of the parameters for which values are determined at stage 1590 maynot be set parameters or targets whose achieving by actual performanceof the air vehicle should be attempted or desired, but rather parametersthat define ends of set or permitted ranges (e.g. of a determinedenvelope that ultimately permits deceleration to substantial hover at apredetermined hover destination position). For example, the determiningof stage 1590 may include determining set pitch, but may also include(in addition to or alternatively) defining a range of permitted pitchangles, wherein deviation from this range results in immediate actionstaken to counter this pitching deviation.

It is noted that stage 1590 may be carried out before and/or during anyother stage of method 1500 for which such parameters may be used (e.g.values of parameters that pertain to the controlling of the operation ofthe at least one tiltable propulsion unit to provide thrust in thegeneral vertical thrust vector direction for providing lift to the airvehicle in stage 1540 may be determined prior to the carrying out ofstage 1540 and/or in parallel to it). It is noted that the determiningof such values may be initiated in response to an initiation of any oneor more stages of method 1500, but this is not necessarily so and suchvalues may be also determined, for example, routinely during the flightof the air vehicle.

Reducing of the difference between the measured groundspeed and the setgroundspeed may be achieved by either increasing or decreasing theairspeed of the air vehicle (and/or by modifying a direction thereof).While in many of the wind conditions that may occur in the duration inwhich stage 1550 is carried out, the airspeed of the air vehicle shouldbe reduced, is it clear that in at least some conditions, increasing theairspeed may be set for reducing the difference between the measuredgroundspeed and the set one.

Decreasing of the airspeed when in the second flight mode has beenexemplified above. Increasing of the airspeed when in the second flightmode—as well as control of such an increasing—may be carried out inseveral ways, possibly in different implementations. For example, thethrust of any of the at least one tiltable propulsion unit may bedirected somewhat forwards (which may involve tilting of that tiltablepropulsion unit, but not necessarily so); thrust may be generated byanother non-tiltable propulsion unit capable of producing at least partof its thrust that is directed so a non-zero component of which isdirected in the general longitudinal thrust vector direction;gravitational potential energy may be converted into kinetic energy(i.e. losing altitude may be used for increasing the airspeed of the airvehicle), and so forth.

FIG. 5 is a graph which illustratively exemplifies restriction of thrustpower based on the measured airspeed of the air vehicle, according to anembodiment of the invention. The abscissa (the X axis) representspossible measured airspeeds of the air vehicle. The airspeed axisrepresented starts at zero measured groundspeed, but it is noted thatits starting point may differ—and thresholds may also be given, forexample, for negative airspeeds (i.e. when the airmass around the airvehicle progresses towards a front of the air vehicle with respect tothe air vehicle, and not vice versa as is normally encountered inforward flight).

The ordinate (the Y axis) represents the thrust in the general verticalthrust vector direction. In the illustrated graph, the beginning of theordinate is equivalent to zero thrust in the general vertical thrustvector direction, but this is not necessarily so. It is noted thatneither units nor values are provided with respect to any of these axes,because the actual thresholds vary significantly depending onimplementation. For example, the thrust provided in a given airspeed fora 70 Kg tiltrotor UAV is substantially lower than the thrust provided inthe same airspeed for a 15 tons tiltrotor multi-mission aircraft.Likewise, the thrust values also greatly depend on other factors such asaerodynamic design, wing area, etc., and potentially also on operationalcircumstances (e.g. the thrust thresholds in a low height may bestricter than in high elevation).

Higher threshold 1710 represents the maximal permitted thrust in anypossible measured airspeed, and lower threshold 1720 represents theminimal permitted threshold in any possible measured airspeed. Line 1730represents a possible thrust during an exemplary descent course of theair vehicle. It is noted that different thrusts are shown in line 1730for a single airspeed, as may occur in different situations—e.g. due towinds or due to the fact that the deceleration is not necessarilymonotonic, and therefore the air vehicle may measure the same airspeedmore than once.

It is noted that while the ordinate (the Y axis) represents the thrustin the general vertical thrust vector direction, the actual thresholds(both lower and higher) may not necessarily be a thrust threshold, butrather may be thresholds of one or more parameters associated withthrust (especially with thrust in the general vertical thrust vectordirection).

For example, the threshold may be a threshold of the power provided (orsourced) by the at least one tiltable propulsion unit for providingthrust in the general vertical thrust vector direction, and/or mayrelate to a state of a throttle controlling that at least one tiltablepropulsion unit. The threshold may also be a threshold that pertains toa measured value measured by a sensor (e.g. sensor measuring a rotatingspeed of a ducted fan or of the speed of downwards airflow created byit).

A controlled value (e.g. throttle state) or a measured value (e.g.measured rotation speed) may be easier to control and keep within anallowed limit than controlling the thrust directly. It is noted thatcontrolling of the thrust power may be implemented by controlling such acontrolled parameter (such as the power provided to the tiltablepropulsion unit or the state of the throttle).

The controlling of the thrust of the at least one tiltable propulsionunit for reducing the difference between the measured and the setgroundspeeds may therefore be restricted in light of the high thresholdand/or of the low threshold—increasing of the thrust power may berestricted based on the higher threshold that is determined in responseto the measured airspeed of the air vehicle (illustrated by line 1710)while reduction of the thrust power may be restricted based on the lowerthreshold that is determined in response to the measured airspeed(illustrated by line 1720).

Increasing of the thrust above the high threshold may result, forexample, in an undesired ascent of the airplane or exceed the boundariesof an energy spending regime. On the other hand, decreasing of thethrust below the lower threshold may result, for example, in hitting theground (if flying at a low altitude) and/or in stalling.

Decreasing of the thrust when hovering or when in low speed may resultin sinking of the air vehicle and in an increase in the load on thewing. Exceeding below the allowed range of thrust may therefore resultin sinking of the air vehicle, and in stalling of the wing. Notably,below a given velocity (which naturally depends on the specificimplementation of the invention, e.g. 15 knots) the stalling of the winghas limited or no detrimental implications, as the greater part of liftis provided by the engines, and the affect of the wind is marginal.

If, however, the velocity of the air vehicle is higher than that givenvelocity but is nevertheless lower than the stalling velocity of the airvehicle, exceeding of the thrust below the allowed range would result anoticeable stalling, and may even result in flipping of the air vehicle.The allowed range, between thresholds 1710 and 1720, therefore defines asafe range in which the lift provided by the wing together with theengines is sufficiently positive.

Since the above exemplified possible results of decreasing the thrustbelow the lower threshold may be much more critical than the aboveexemplified possible results of increasing the thrust above the higherthreshold, it is noted that exceeding the high threshold may sometimesbe tolerated (e.g. in view of other aerodynamic considerations), whilereducing thrust below the lower threshold may be more strictly avoided(and even entirely prohibited).

While not illustrated, it is noted that the rate of reducing the thrustmay also be limited. For example, even if the high threshold is exceededfor whatever reason, the thrust according to such an implementation maynot be sharply cut or reduced, but is rather reduced in a controlledmanner wherein the reduction rate does not exceed a predetermined value.This may be implemented in order to keep drastic and too quick loss ofthrust that may lead to instability, crashing, or other unset results.The increasing rate of the thrust may or may not be similarly limited.

As aforementioned, stage 1560, that is carried out during at least partof stage 1550 (and possibly also at other times in which parts of method1500 are carried out) includes restricting reduction of the thrust powerbased on the lower threshold that is determined in response to ameasured airspeed of the air vehicle.

While not necessarily during every moment of stage 1550, it is notedthat possibly, during at least some parts of this stage a controlling ofa reduction of the thrust power of the at least one tiltable propulsionunit will be carried out.

During the slowing down (the reduction of the groundspeed of the airvehicle), the one or more wings of the air vehicle (as well aspotentially other aerodynamic components of which, such as ailerons, ifimplemented) still produce lift. Slowing down of fixed-wing airplanesusually involves reducing of thrust in the general longitudinal thrustvector direction. An additional and important technique used for slowingfixed-wing airplanes involves pitching a nose of such an airplane up,thus creating more drag. It is noteworthy that prior art rotary-wingaircraft (such as helicopters) may also tilt back the rotor for slowingdown, which usually co-occurs with a pitching up of the aircraft.

Since when in the second flight mode, the air vehicle of method 1500incorporates both a fixed wing and a rotary wing, the controlling of thereducing of the groundspeed (by way of reducing airspeed) may indeedinclude controlling a pitching up of the air vehicle (e.g. bycontrolling aerodynamic components of which, e.g. as discussed above,wherein it should however be noted that modifying a direction of thethrust provided by any of the at least one tiltable propulsion unitsdoes not necessarily involve tilting the entire vehicle, as in someimplementations the at least one tiltable propulsion unit may be tiltedbackwards to achieve this effect).

Pitching the air vehicle up (raising its nose) results in increment inthe lift generated due to the at least one fixed wing of the airvehicle, and therefore may require lift compensation that involvesreducing the thrust in the general vertical thrust vector direction(e.g. generated by the at least one tiltable propulsion unit). Pitchingthe air vehicle backwards when the thrust by the at least one tiltablepropulsion unit is directed in the general vertical thrust vectordirection without such compensation may result in excess thrust beinggenerated and ascent of the air vehicle.

Method 1500 may also include keeping a pitch of the air vehicle within apermitted pitch range concurrently with at least a part of thecontrolling of the reducing of the groundspeed, wherein the permittedpitch range is dynamically determined in response to the measuredairspeed of the air vehicle. It is noted that maintaining the pitchwithin a permitted pitch range may also be carried out concurrently withat least part of the controlling of the descending course. Thedynamically determined permitted pitch range may be determined based onother parameters except the airspeed of the air vehicle. For example—itmay be further determined based on a part of the course (e.g.descending/non-descending), on an orientation of the at least onetiltable propulsion unit, on ambient aerodynamic conditions (e.g. wind,weather), on height, etc. Maintaining the pitch within the permittedpitch range may be used to prevent stalling of the air vehicle.

FIG. 6 is a graph which illustratively exemplifies restriction of thepitch of the air vehicle based on the measured airspeed thereof,according to an embodiment of the invention. The abscissa (the X axis)represents possible measured airspeeds of the air vehicle. The airspeedaxis represented starts at zero measured groundspeed, but it is notedthat its starting point may differ—and thresholds may also be given, forexample, for negative airspeeds (i.e. when the airmass around the airvehicle progresses towards a front of the air vehicle with respect tothe air vehicle, and not vice versa as is normally encountered inforward flight).

The ordinate (the Y axis) represents pitch of the air vehicle withrespect to the horizon. In the illustrated graph, the beginning of theordinate is equivalent to zero pitch (may be defined, for example, thata line connecting the nose of the air vehicle and the rearmost partthereof is substantially horizontal), but this is not necessarily so.

It is noted that neither units nor values are provided with respect toany of these axes, because the actual thresholds vary significantlydepending on implementation. For example, the pitch permitted in a givenairspeed for a 70 Kg tiltrotor UAV may be substantially different thanthe pitch permitted in the same airspeed for a 15 ton tiltrotormulti-mission aircraft. Likewise, the permitted pitch values alsogreatly depend on other factors such as aerodynamic design, wing area,etc., and potentially also on operational circumstances (e.g. the pitchthresholds in a low height may be stricter than in high elevation).

The permitted pitch range at any airspeed may be defined by a higherpitch threshold and a lower pitch threshold. Higher pitch threshold 1810represents the maximal permitted pitch in any possible measuredairspeed, and lower pitch threshold 1820 represents the minimalpermitted pitch in any possible measured airspeed. Line 1830 representsa possible pitch during an exemplary descent course of the air vehicle.It is noted that different pitches are shown in line 1830 for singleairspeed, as may occur in different situations—e.g. due to winds or tothe fact that the deceleration is not necessarily monotonic, andtherefore the air vehicle may measure the same airspeed more than once.

It is noted that while the ordinate (the Y axis) represents the pitch inthe general vertical pitch vector direction, the actual pitch thresholds(both lower and higher) may not necessarily be a pitch threshold, butrather may be thresholds of one or more parameters associated withpitch. For example, the pitch threshold may be a threshold of a pitchmeter, an inclination meter threshold, and so forth. A controlled valueor a measured value may be easier to control and keep within allowedlimit than controlling the pitch directly. It is noted that thecontrolling of the pitch may be implemented by controlling one or moreoperations of one or more pitch influential components of the airvehicle—such as by controlling a inclination angle of the at least onetiltable propulsion unit, or modifying a state of a aerodynamicsubsystems of the air vehicle, such as its elevators.

The controlling of the thrust of the at least one tiltable propulsionunits for reducing the difference between the measured and the setgroundspeeds—or controlling of other parameters—may therefore berestricted in light of the high pitch threshold and/or of the low pitchthreshold, and vice versa.

Increasing of the pitch above the high pitch threshold may result, forexample, in exceeding a stall angle, and decreasing of the pitch belowthe lower pitch threshold may result, for example, in reducing theability of the air vehicle to lose speed.

It should be noted that at least part of the controlling of the reducingof the groundspeed may possibly be carried out after the controlling ofthe descending course of the air vehicle (i.e. while stage 1550 may atleast partly coincide with stage 1540, according to such animplementation at least part of stage 1550 is carried out after stage1540) during a substantially horizontal flight of the air vehicle(denoted stage 1552).

Clearly, other types of limitations and thresholds may also beimplemented—either in addition or instead of the aforementioned thrustand/or pitch limits. For example, the angle of the air vehicle (e.g. itsmain axis connecting it frontmost and rearmost parts) with respect towind, may be limited. In some implementations of the invention, when theair vehicle slows down, especially when in the second flight mode, itmay have a tendency to rotate so it faces the wind. Limits may be usedfor limiting that angle, and this may assist in efficiently bringing theair vehicle to its destination.

Referring to the example set forth in FIG. 4A, stage 1550 may be carriedout at part 1015 of course 1010. It should be noted that a flightdirection of the air vehicle during the substantially horizontal flightis not necessarily exactly horizontal, and that deviations from aperfect horizontal flight may occur. The substantially horizontal flightmay be restricted to an altitude range that is fixed over time, andwhich thus may be considered as defining an imaginary horizontalcorridor in which the air vehicle flies.

Method 1500 may therefore include controlling altitude modification ofthe air vehicle during the substantial horizontal flight for keeping thealtitude of the air vehicle between a lower altitude threshold and ahigher altitude threshold, wherein this controlling of altitudemodification may be carried out concurrently with the controlling ofstage 1550. While not necessarily so, the lower and the higher altitudethreshold may be kept constant during the aforementioned controlling ofthe altitude modification, but this is not necessarily so.Alternatively, this may also be implemented, for example, by controllingaltitude modification of the air vehicle for minimizing a verticaldeviation of the air vehicle from a set altitude.

Such minimizing of maintaining altitude may be carried out during thesubstantial horizontal flight but also during other parts of the method,and may be carried out at least partly concurrently with the controllingof the reducing of the groundspeed.

In an example, the lower altitude threshold may be kept at 5 metersabove local ground level (or an estimated/average ground level) and thehigher altitude threshold may be kept at 10 meters above local groundlevel. Clearly, other values may be used, as would be clear to a personwho is of skill in the art, e.g. depending on dimensions and aerodynamiccapabilities of the implemented aircraft, on environmental conditions,on tactical/operational considerations, and so forth. Furthermore, thealtitude thresholds are not necessarily defined in relation to a localground height, and may be defined in relation to any other knownaltitude value, e.g. above sea level, above a destination hoveringaltitude, and so forth. Where applicable, e.g. in cases where thresholdsare defined above local ground level(real/measured/estimated/average/etc.) and the ground level is nothorizontal, the condition pertaining to the substantial horizontalflight may be replaced with a condition pertaining to flight at asubstantially fixed height above local ground level.

It is noted that while controlling altitude modification in substantialhorizontal flight, the controlling may be responsive to thresholds thatare not fixed. For example, the thresholds may be somewhat modifieddepending on altitude modifications themselves (e.g. if the air vehiclerose during that controlling, whether intentionally or unintentionally,the threshold may be updated to reflect this change). The conditionpertaining to the substantial horizontal condition may be replaced witha condition pertaining to flight with a relatively low rate of altitudechange (e.g. course having an altitude change rate of less than 1%, ofless than 2%, etc.).

Flying in a substantially horizontal flight (or other equivalents, e.g.such as those aforementioned) is not mandatory in every implementationof the invention, and a controlling system that at least partly controlsthe air vehicle may determine whether such a substantially horizontalcourse is set at the stage of groundspeed reduction (which may be thefinal stage prior to hovering). Some exemplary considerations fordeciding upon such a substantially horizontal flight course are:aerodynamic considerations; payload considerations (e.g. whenaffectivity of the payload increases substantially at the designatedhovering altitude), reducing exposure of the air vehicle to externalthreats, lowering below cloud level, and so on.

It is noted that in some situations, method 1500 may include restrictingof different parameters relating to the state of the air vehicle. Someexamples have been offered above. For example, as discussed above, themethod may include controlling the altitude modification of the airvehicle for maintaining the altitude of the air vehicle between a loweraltitude threshold and a higher altitude threshold (especially duringthe substantial horizontal flight, but possibly also before that, e.g.during the descent of the air vehicle). As further discussed above,method 1560 may also include controlling thrust power of the at leastone tiltable propulsion unit while restricting reduction of the thrustpower based on a lower threshold that is determined in response to ameasured airspeed of the air vehicle. The controlling of altitudemodification may be carried out concurrently with the controlling of thethrust.

In some scenarios, attempting to control more than single parameters maylead to situations in which the restricting of a first parameterconflicts with the restricting of another parameter. For example,keeping the air vehicle within a permitted altitude range may require atsome point exceeding the threshold restrictions applied. For example, ifa sudden gust of wind results in a sudden increase of lift from thewind, a fast reduction of the thrust in the general vertical thrustvector direction may be required in order to avoid rising above thehigher altitude threshold. Therefore, at least one of the two respectivethresholds must be exceeded, and in different implementations, differentorder rules may determine favoring one type of threshold over another.

Method 1500 may include minimizing a vertical deviation of the airvehicle from a set altitude (e.g. by the attempt to keep the altitude ofthe air vehicle between a lower altitude threshold and a higher altitudethreshold), concurrently with at least a part of the controlling of thereducing of the groundspeed, wherein the minimizing is restricted atleast by the restricting of the reduction of the thrust power based onthe lower threshold.

That is, according to such an implementation, preventing of a reductionof the thrust power below the lower threshold is prioritized over theminimizing of the vertical deviation from the set altitude, and reducingthe thrust in a way that would result in exceeding the lower threshold(e.g. by lowering a rotation rate of the blades of a tiltable rotor thatis the tiltable propulsion unit below a lowest permitted rotation ratefor the current measured airspeed) is prevented, even at the cost offailing to minimize the vertical deviation from the set altitude (e.g.even if it results in exceeding the concurrent higher altitudethreshold).

The minimizing of the vertical deviation may be restricted (possiblyfurther restricted, if implementing the immediately above disclosedrestriction) by restricting a rate of thrust power reduction based on amaximal permitted thrust power reduction rate. That is, even ifreduction of the thrust for minimizing the vertical deviation does notrequire thrust reduction that would result in exceeding of the lowerthreshold, it is nevertheless limited in such an implementation by themaximal permitted thrust power reduction rate.

The thrust power reduction rate may pertain directly to the thrust, andmay also pertain to a reduction rate of a controlled value (e.g.throttle state) or a measured value (e.g. measured rotation speed). Forexample, even if the air vehicle is about to exceed the higher altitudethreshold, it is determined that the thrust should be reduced, and thethrust-related lower threshold was not exceeded, the rate in which thethrust may be reduced in such an implementation is restricted by themaximal permitted thrust power reduction rate. The maximal permittedthrust power reduction rate may be defined in different ways, e.g. in arelative fashion (for example, a reduction of less than 1% of thecurrent value per second) or in a fixed way (a reduction of less than200 rpm per minute).

As aforementioned, at least part of the controlling of the reducing ofthe groundspeed may possibly be carried out after the controlling of thedescending course of the air vehicle, during a substantially horizontalflight of the air vehicle (denoted stage 1552). It is nevertheless notedthat at least part of the controlling of the reducing of the groundspeedmay possibly be carried out concurrently to controlling of thedescending course of the air vehicle, and prior to its substantiallyhorizontal flight—if at all implemented. Referring to FIG. 4B, forexample, part 1024 is the part of course 1020 in which the controllingof the reducing of the groundspeed substantially to a hover is carriedout (while the at least one tiltable propulsion unit provides thrust inthe general vertical thrust vector direction). It can be seen that part1024 of the course partly overlaps with part 1023 of course 1020 inwhich descent of the air vehicle is carried out and controlled.

Moreover, possibly substantial reduction of the groundspeed may becarried out during the descent, and not only in the horizontal flight(if at all implemented). Such a reduction of groundspeed may be carriedout when the at least one tiltable propulsion unit provides thrust inthe general vertical thrust vector direction, but may also includesubstantial reduction of groundspeed prior to the tilting of the atleast one tiltable propulsion unit, when the at least one tiltablepropulsion unit provides thrust in the general longitudinal thrustvector direction.

Method 1500 may also include controlling, during at least the secondpart of the descent, reduction of the groundspeed to a fraction of theinitial groundspeed at the beginning of the descending course (e.g. afraction from a cruise speed of the air vehicle in which it cruisedprior to starting to descent) before an end of the descent and beforethe beginning of the substantially horizontal flight of an air vehicle.In the course of FIG. 7, for example, if the cruising speed of the airvehicle is 35 knots (35 kts) and under the presumption of no significantwind, the initial groundspeed may be considered as 35 kts. At the end ofthe descent, prior to the horizontal flight, the groundspeed of the airvehicle may be for example 25 kts. In such an example, the reduction ofthe groundspeed is to about 70% of the initial groundspeed. Theremaining 70% may be reduced during the substantially horizontal flight.In various implementations, the groundspeed may be reduced to otherfractions of its initial value—e.g. about 50%, about 60%, about 70%,about 80% and about 90%.

The reduction of the groundspeed prior to substantially horizontalflight may also be governed otherwise. Method 1500 may includecontrolling, during at least the second part of the descent, reductionof the groundspeed below a maximum permitted threshold speed before theend of the descent and before the beginning of the substantiallyhorizontal flight of the air vehicle.

The transition from descending course flight to horizontal flight may becarried out gradually, e.g. as a flare sub-stage (as indicated in FIG.7). This stage may be the part of the descending course in which thelarger part of the descent airspeed reducing is carried out (see, forexample, the airspeed values indicated in FIG. 7).

It is noted that failure to sufficiently reduce the groundspeed (eitherprior to the end of the descent or at different stages throughout it orafter it) may result in cancellation of the entire landing/slowing to ahover process, e.g. as the remaining distance until the destination maybe insufficient to fully stop in such conditions.

It is noted that another scenario, in which the air vehicle slows downtoo quickly may also be an undesired one. If the air vehicle slows downat a too early stage (e.g. below 5 kts in the regime exemplified in FIG.7), it may take a very long time to reach its destination, thus alsocausing waste of a great deal of energy on the way. All the more so,accelerating of the air vehicle once its velocity is so greatly reducedmay be costly, inefficient, and time consuming.

The controlling of the reducing of the groundspeed may possibly includepreventing a reduction of the airspeed below a predetermined thresholduntil reaching a vicinity of the predetermined hover destinationposition. This may be achieved, for example, at least by controlling apitch angle of the air vehicle.

As was demonstrated above, in some scenarios, the air vehicle may reacha state in which it cannot stop in the predetermined hover destinationposition, or it cannot do so in reasonable time or using a reasonableamount of energy. Even if it may possibly do so, certain decision rulesmay be implemented to determine that the chances of completing such atask reasonably are below an accepted threshold.

Method 1500 may also include stage 1580 of repeatedly checking whetherflight parameters are within an envelope permitting deceleration tohover at a predetermined hover destination position. Such repeatedchecking may be carried out at regular or irregular intervals, and eachinstance may be triggered by time or based on one or more measuredparameters.

The repeatedly checking may commence, for example, during the first partof the descent and at least until reduction of the groundspeed tosubstantial hovering. By way of example, the repeated checking may becarried out at least until a first occurrence of an event selected from(a) receiving a negative result and (b) reduction of the groundspeed tosubstantial hovering.

If a negative result is received in the checking (i.e. if at least oneof the flight parameters exceeds such an envelope), the method mayinclude either an aborting of the landing/slowing down to a hoverprocess, but may also include reiterating some parts of the method—e.g.by returning to the first flight mode, re-entering the envelope, andgoing through the sequence of stages (1510 to 1550) to a successfulresult.

It should be noted that even if the result of the checking is negative,the response is not necessarily ascending and reattempting to land (orotherwise to descend). For example, if, following the negative result,it is determined that the air vehicle is closer to landing than toflying (e.g. its airspeed is very low), then even if landing in thedestination is not feasible, the method may nevertheless continue withlanding the air vehicle in a near location. Other responses to anegative result of the checking may also be implemented, and variousdecision rules may be implemented to determine among a plurality of suchpossible response.

Method 1500 may include selectively instructing tilting of the at leastone tiltable propulsion unit to provide thrust in the generallongitudinal thrust vector direction if a result of the checking wasnegative, and controlling a directing of the air vehicle to a positionand a state in which the flight parameters are within the envelope, andreinitiating the method, starting again with the controlling during thefirst part of the descent.

Optional stage 1570 includes controlling a horizontal progressiondirection of the air vehicle. It is noted that the horizontalprogression direction may be kept substantially uniform during theentire course taken by the air vehicle (e.g. the entire course 1010 or1020), but this is not necessarily so. For example, part of the course(e.g. the descent) may be carried out in substantially direct horizontalprogression direction, while another part may include controllingturning of the course and modifying the horizontal progressiondirection. The controlling of stage 1570 may be carried out inparallel—or partly in parallel, to some or all of the previouslydisclosed stages.

The controlling of the horizontal progression direction may be carriedout in response to a set horizontal progression direction—that may befixed or changed from time to time. The determining of such a sethorizontal progression direction may be based on variousconsiderations—such as geometric considerations (e.g. distance from thedestination location and current altitude), atmospheric conditions (e.g.wind), aerodynamic and/or energetic efficiency, operationalconsiderations, capabilities of the air vehicle, and so forth.

While some parameters may be determined by a system (or person)implementing method 1500 (e.g. in stage 1590 thereof), some parametersmay be determined to the system by another system, module, or person. Incertain examples, some such parameters that may be defined before thecontrolling of method 1500 are as follows:

-   -   a. Where should the air vehicle land or hover? (e.g. what is the        hover destination position? What is the final landing        destination?)    -   b. At what height should the air vehicle hover?    -   c. From which direction should the air vehicle arrive? (e.g.        against the wind, at an azimuth of 271°, when a captured image        by a camera mounted on the air vehicle matches one or more        reference images, etc.)    -   d. At what access angle should the air vehicle descend?    -   e. What are the horizontal ranges allocated to some or all of        the different sub-stages?    -   f. What are the timing constrains for the landing?

While not necessarily all of these parameters are determined in advance,it is possible that all of them (or any sub-combination thereof, e.g.including parameters a, b, c, d, and e but not f, and so forth) aredetermined in advance. The controlling of the different stages (1510through 1550, and 1570) may depend directly on these parameters (or someof them, depending on the stage), but may also depend on parametersdetermined in stage 1590 based on these parameters. It is noted thatthese parameters—or parameters similar thereto—may also be determinedfor the various stages of method 1600 described below.

FIG. 3B is a flow chart of method 1600 for controlling a substantialvertical descent of an air vehicle, according to an embodiment of theinvention. The air vehicle of method 1600 may include at least onetiltable propulsion unit, and may be the air vehicle of method 1500. Itwill however be clear to a person who is of skill in the art that method1600 may be implemented for other types of air vehicles as well, such ashelicopters.

Stage 1550 includes controlling of the reducing of the groundspeed ofthe air vehicle of method 1500 substantially to a hover, and may befollowed by the stages of method 1600 that includes controlling asubstantially vertical descent of an air vehicle—potentially until ithas landed. However, method 1600 may also be implemented independentlyof method 1500—e.g. if the air vehicle was brought to a substantialhover in another way. In an example in which method 1600 is carried outafter method 1500, the controlling of the vertical descent of the airvehicle may be carried out during part 1017 of course 1010 illustratedin FIG. 4A. The vertical descent illustrated by part 1017 may end with alanding on the ground (denoted 1018).

Generally, method 1600 may include controlling a substantially verticaldescent of the air vehicle, after the reducing of the groundspeed of theair vehicle substantially to hover, until a fulfillment of at least oneground detection condition. Alternatively, the controlling of thesubstantially vertical descent may be continued until a fulfillment ofanother decision rule—such as reaching a predetermined hovering heightthat is lower than a hovering height in which the air vehiclesubstantially hovered prior to the substantially vertical descent. Thecontrolling of the substantially vertical descent may be implemented bycarrying out method 1600, but this is not necessarily so.

It is noted that method 1600 includes several stages of controlling, asis disclosed below in detail. Such controlling may be implemented invarious ways. Such controlling may be implemented by a pilot, by anotherperson onboard, or by a remote human operator (e.g. for an unmanned tiltrotor air vehicle). However, method 1600 may also be implemented by oneor more computerized systems (e.g. as exemplified in relation to system1200). Such a system may be mounted onboard the air vehicle of method1600, or externally, and multiple such systems may coordinate toimplement method 1600 (wherein each stage of the method may beimplemented by a single system or a combination of such computerizedsystems). Additionally, a combination of one or more human controllersand one or more computerized systems may also be implemented.

According to an embodiment of the invention, the controlling of thedescending course and the controlling of the reducing of the groundspeed(both of which are disclosed in greater detail below) include automatedcontrolling by at least one processor of a control unit mounted on theair vehicle. It is noted that such processors and/or other computerizedsystems may be a dedicated system (implemented in hardware, firmware,etc.), and may also be implemented in software run by a processor ofanother system mounted on the air vehicle.

It is also noted that different stages of method 1600 includecontrolling (e.g. controlling a reduction of an altitude of the airvehicle in stage 1620). While not necessarily so, in each of thecontrolling stages method 1600 may possibly also include the carryingout of the controlled operation, even if not explicitly elaborated so.Continuing the same example, in addition to the controlling of stage1620, method 1600 may further include reducing the altitude of the airvehicle.

Stage 1610 of method 1600 includes initiating the controlling of thesubstantially vertical descent of the air vehicle. The initiating of thecontrolling of the substantial vertical descent may be carried out basedon different parameters, e.g. any one or more of the followingparameters—determining that the aircraft reached a substantial hover,determining that it arrived at the destination location, a state of theairplane (e.g. remaining energy level), time elapsed from initiation ofthe controlling of the descent or any other identified stage, and so on.Referring to the examples set forth in the previous drawings, stage 1610may be carried out by a control unit such as control unit 1220.

Stage 1610 may include stage 1612 of initiating the controlling of thesubstantially vertical descent of the air vehicle if predetermined timeelapsed from the tilting of the at least one tiltable propulsion unit,regardless of sensors data. That is, even if the sensors data neitherindicate that the groundspeed of the air vehicle was reduced tosubstantial hover, nor that the air vehicle arrived at the destinationlocation (or another expected condition is not met), the controlling ofthe substantially vertical descent may still be initiated, in view ofthe time elapsed. Of course, stage 1612 is not implemented if the airvehicle of method 1600 does not include a tiltable propulsion unit.

It is noted that the time reference of stage 1612 may be the time of thetilting (as indicated above), but may also be replaced by (or be inaddition to) a time reference pertaining to any of the other stages ofmethod 1500—e.g. the time in which the substantially horizontal flightof stage 1552 began.

The initiation of the controlling of the substantially vertical descent,regardless of sensors data, may be implemented for several reasons. Forexample, the sensors data may be faulty (e.g. an airspeed sensor or alocation sensor may be faulty). In another example, such initiation,irrespective of the sensors data, may be implemented for savingenergy—e.g., even if the air vehicle did not reach the destinationlocation, doing so may take too much time and energy (e.g. due to astrong headwind), so that landing after a predetermined time (or, whenmeasuring that energy levels dropped below a predetermined level) may beimplemented. In yet another example, it is noted that a preceding stagemay not have been completed correctly (e.g. there was an error in thenavigation of the air vehicle in a preceding stage), and therefore otherstop-conditions may not be met ever in some scenarios.

Referring to the stages of method 1500, it is noted that initiation ofdifferent stages may be determined based on sensors data (e.g.initiation of the substantially horizontal flight in optional stage 1552may be based on an altitude meter), but may also be determined based onthe time elapsed from any predetermined point in time (e.g. from theinitiation of one or more of the preceding stages).

Stage 1620 of method 1600 includes controlling a reduction of analtitude of the air vehicle. The controlling of the reduction of thealtitude may include controlling various aerodynamic subsystems of theair vehicle. It may include controlling the thrust of one or morepropulsion units directed in the general vertical thrust vectordirection (e.g. a tiltable propulsion unit, a non-tiltable propulsionunit). It may also include controlling of additional aerodynamicsubsystems of the air vehicle (such as those exemplified above), e.g.for controlling yaw, pitch, and/or roll of the air vehicle during thealtitude reduction.

It should be noted that the course of the air vehicle during the descentof stage 1620 is not necessarily a strictly monotonic descending one,and that while an altitude of the air vehicle at an end of the descentis substantially lower than its altitude at a beginning of the descent,the air vehicle may nevertheless experience some temporary ascents (e.g.due to unexpected winds or air conditions, due to moving of controlsurfaces of the air vehicle, and even as effects of actions taken aspart of controlling another aspect of the air vehicle—e.g. in order tokeep the air vehicle within an envelope that ultimately permits landingof the air vehicle).

Controlling of the reduction of the altitude of the air vehicle may beachieved at least by controlling an operation of one or more of theaerodynamic subsystems of the air vehicle. Such parts may include, byway of example, the at least one tiltable propulsion unit, at least onenon-tiltable propulsion unit, a throttle, an engine, ailerons,elevators, rudder, ruddervator, flaperons, elevons, wing flaps, slats,spoilers, air brakes, variable-sweep wings, non-tiltable propulsionunit, blades of rotors, and so on. It is noted that different stages ofmethod 1600 (e.g. the controlling of stage 1620) may include controllingan operation of at least one aerodynamic subsystem of the air vehicleselected from a group consisting of an aileron, an elevator, a rudder, aruddervator, a flaperon, elevons, and a wing flap.

The controlling of stage 1620 may include controlling modification ofthrust power of such a propulsion unit (e.g. the at least one tiltablepropulsion unit) for minimizing a deviation of the monitored altitude ofthe air vehicle from a dynamic set altitude that is changed from time totime. The set altitude may be modified continuously, but not necessarilyso. Possibly, the set altitude may be a monotonically decreasing setaltitude, and is potentially monotonically decreasing in a constant rate(e.g. 0.2-0.6 meters per second). The groundspeed of the air vehicleduring altitude reduction (and/or during touchdown, if implemented) maybe restricted, e.g. to under 2 kts.

It is noted that during stage 1620, stage 1622 may also be carried out,including keeping a location of the air vehicle. The keeping of thelocation may be made with respect to local ground (e.g. using a camera,a RADAR, etc.), or to other location indicative means—such as aninertial navigation system and/or a GPS system. The keeping of thelocation may be responsive to a measured location (e.g. based on GPS) orin response to measured groundspeed (e.g. GPS and/or Doppler). It isnoted that those two do not always coincide.

Stage 1620 may be implemented as optional stage 1624 that includescontrolling modification of thrust power of the at least one tiltablepropulsion unit for minimizing a deviation of the monitored altitude ofthe air vehicle from a monotonically decreasing set altitude.

Stage 1620 may be implemented as optional stage 1626 that includescontrolling modification of thrust power of the at least one tiltablepropulsion unit for minimizing a deviation of the monitored altitude ofthe air vehicle from a dynamic set altitude that decreases below a localground level. Having the set altitude decrease below the local groundlevel may be used in scenarios of landing, in which the air vehicle iscontrollably lowered to local ground level.

When attempting to minimize the deviation of the monitored altitude fromthe set altitude, if the monitored altitude is significantly higher thanthe set altitude at a given moment, the controlling may include reducingthrust level of the propulsion unit (and vice versa). Reduction of thethrust level may result in lowering of the monitored altitude—and thusin a reduction of the altitude deviation.

If the set altitude is set to a level that is sufficiently lower thanthe local ground level, then even if the monitored altitude is measuredwith some errors, it is likely to be higher than the set altitude. Thatis—even if the air vehicle touched the ground without a proper sensoridentifying it, the thrust of the propulsion unit will nevertheless bereduced, due to deviation between the local ground level measuredaltitude and the set altitude that is sufficiently lower therefrom. Allthe more so, if the set altitude is monotonically decreased, thereducing of the thrust power of the at least one propulsion unit may begradual.

It is noted that the thrust levels are not necessarily restricted asexemplified in FIG. 5 (even though the decreasing rate of the thrust maynevertheless still be restricted), and therefore such lowering of theset altitude may result in the gradual decreasing of the thrust power tozero. The degree to which the set altitude goes below the local groundlevel may depend on various parameters.

Such parameters may include, for example, expected errors in one or moreof: the measured altitude, the local ground altitude; the lowering rate(e.g. depending on the permitted thrust reduction rate), the externalconditions, the physical dimensions of the air vehicle, the strength ofthe air vehicle, and so forth. For example, the set altitude may belower than the local ground level altitude, more than five times theheight dimension of the air vehicle, more than ten times that height ofthe air vehicle, more than twenty times that height, and the like.

Stage 1620 may be implemented as controlling modification of thrustpower of the at least one tiltable propulsion unit for minimizing thedeviation of the monitored altitude of the air vehicle from amonotonically decreasing set altitude that decreases below the localground level more than five times the height of the air vehicle.

Even if this mechanism is implemented as a safety mechanism, othermechanisms may also be implemented for determining when to stop thecontrollable reduction of altitude—e.g. when the air vehicle isdetermined to have reached a destination height and/or has landed.

Stage 1630 of method 1600 includes determining whether an altitudereduction stop condition was met. Such an altitude reduction stopcondition may be a ground detection condition, but this is notnecessarily so, and other altitude reduction stop conditions may beimplemented—in addition to or instead of a ground detection condition.

Some altitude reduction stop conditions which may be implemented invarious implementations of the invention are:

-   -   1. Did an input from a ground detection sensor (whether        dedicated to ground detection or not) indicate detection of        ground? Such a ground detection sensor may be, for example, an        acceleration sensor, a RADAR altimeter; an ultrasonic sensor, a        laser sensor, an optical sensor (e.g. a camera), a RADAR, wheels        pressure sensor (or other landing gear pressure sensor), and so        forth.    -   2. Did a predetermined time elapse from the initiating of stage        1610? This may include determining to stop the altitude        reduction if a maximum time threshold elapsed, and may also        include preventing a stopping of the altitude reduction if        insufficient time elapsed (thereby preventing, for example,        unplanned killing of engine when the air vehicle did not yet        land, if a sensor provided an erroneous result).    -   3. Was a thrust level reduced below a predetermined level (e.g.        due to the deviation from the set altitude described above)?

It is noted that the controlling of the modification of the thrust powerin stage 1620 may be based, at least in part, on data gathered from thesensors that also operate as the ground detection sensors.

If the altitude reduction condition (or conditions) is met, the methodmay continue with stopping the controlling of the reduction of thealtitude of the air vehicle (denoted 1640). It may also include stoppingan operation of an engine of the air vehicle or other systemsthereof—especially if the altitude reduction carried out was for landingthe air vehicle.

If the altitude reduction was carried out for hovering at a loweraltitude, method 1600 may continue with controlling maintaining of analtitude and location of the air vehicle. The stopping of the altitudereduction may also be accompanied by an activation of one or more airvehicle systems.

Significantly, stage 1640 may be preceded by triggering an activation ofground detection sensors (and/or triggering of accepting data receivedtherefrom as sufficient for an altitude reduction stop condition). Sincethe ground detecting sensors may not be very accurate and may be proneto errors, activation of those sensors for ground detection may betriggered at a half-way stage during the altitude reduction. Forexample, the sensors may be so activated at a height of one or twometers, at a height equal to the height dimension of the air vehicle ortwice that height, and so forth.

If method 1600 includes stopping of one or more engines of the airvehicle, such stopping may be carried out gradually. In an exemplaryimplementation, during the first 15 seconds of the stopping, thestopping is gradual and slow. After that time the stopping mayaccelerate.

Method 1600 may further include optional stage 1690 of determiningvalues for course setting parameters for the air vehicle, but this isnot necessarily so. Possibly, at least some of the parameters for whichvalues are determined at stage 1690 may not be set parameters or targetswhose achieving by actual performance of the air vehicle should beattempted or desired, but rather parameters that define ends of set orpermitted range (e.g. of a determined envelope that ultimately permitsdeceleration to substantial hover at a predetermined hover destinationposition). For example, the determining of stage 1690 may includedetermining set pitch, but may also include (in addition to oralternatively) defining a range of permitted pitch angles, whereindeviation from this range results in immediate actions taken to counterthis pitching deviation.

It is noted that stage 1690 may be carried out before and/or during anyother stage of method 1600 for which such parameters may be used (e.g.values of parameters that pertain to the controlling of the thrust instage 1626 may be determined prior to the carrying out of stage 1626and/or in parallel to it). It is noted that the determining of suchvalues may be initiated in response to an initiation of any one or morestages of method 1600, but this is not necessarily so and such valuesmay be also determined, for example, routinely during the flight of theair vehicle.

Method 1600 may also include stage 1680 of repeatedly checking whetherflight parameters are within an envelope permitting verticallydescending to a predetermined destination. Such a destination may be alanding destination and may be a hovering destination, depending on theimplemention. The repeated checking may be carried out at regular orirregular intervals, and each instance may be triggered by time or basedon one or more measured parameters.

The repeated checking may start, for example, with the initiating ofstage 1610 and until an end of the vertical descent. By way of example,the repeated checking may be carried out at least until a firstoccurrence of an event selected from (a) receiving a negative result and(b) end of the descent.

If a negative result is received in the checking (i.e. if at least oneof the flight parameters exceeds such an envelope), the method mayinclude either an aborting of the landing/descending to a hover process,but may also include reiterating some parts of the method—e.g. in eitherthe first or the second flight mode, re-entering the envelope, and goingthrough the sequence of stages (starting at 1610) to a successfulresult.

It should be noted that even if the result of the checking is negative,the response is not necessarily trying again to land (or otherwise todescend). For example, another alternative is to reenter flight mode,and to return to the first flight mode.

Referring to both methods 1500 and 1600, it is noted that each of thesemethods may be implemented by using a program storage device readable bymachine, tangibly embodying a computer readable code portion executableby the machine for performing method 1500 and/or method 1600. Differentimplementations of such a program storage device and the program ofinstructions embodied therein may correspond to the variousaforementioned implementations of method 1500 and 1600, even if notexplicitly elaborated. It is noted that control system 1200 may includesuch a program storage device (e.g. database 1230), and may otherwisehave access to such a program storage device.

For example, a program storage device, readable by machine is disclosed,such a program storage device tangibly embodying a computer readablecode portion executable by the machine for controlling a decelerationprocess of an air vehicle which includes at least one tiltablepropulsion unit, each of the at least one tiltable propulsion units istiltable to provide a thrust whose direction is variable at leastbetween a general vertical thrust vector direction and a generallongitudinal thrust vector direction with respect to the air vehicle,the computer readable code portion includes instructions for: (a) duringa descent of the air vehicle, controlling a descending course of the airvehicle based on at least monitored airspeed and monitored altitude ofthe air vehicle, the controlling including carrying out in the followingorder: (i) in a first part of the descent, controlling an operation ofthe at least one tiltable propulsion unit to provide thrust in thegeneral longitudinal thrust vector direction for propelling the airvehicle; and (ii) following a tilting of the at least one tiltablepropulsion unit, controlling in a second part of the descent anoperation of the at least one tiltable propulsion unit to provide thrustin the general vertical thrust vector direction for providing lift tothe air vehicle; and (b) controlling a reducing of a groundspeed of theair vehicle substantially to a hover, while the at least one tiltablepropulsion unit provides thrust in the general vertical thrust vectordirection, at least by controlling thrust power of the at least onetiltable propulsion unit for reducing a difference between measuredgroundspeed of the air vehicle and set groundspeed, while restrictingreduction of the thrust power based on a lower threshold that isdetermined in response to a measured airspeed of the air vehicle.

The instructions for controlling the descending course and theinstructions for controlling the reducing of the groundspeed may includeinstructions for automated controlling by at least one processor of acontrol unit mounted on the air vehicle.

The instructions for controlling of the deceleration process may includecontrolling the deceleration process of the air vehicle that includes awing, and balancing between contradictory aerodynamic effects resultingfrom the wing and from the at least one tiltable propulsion unit.

The instructions for controlling of the reducing of the groundspeed mayinclude instructions for carrying out at least part of the controllingof the reducing of the groundspeed carried out after the controlling ofthe descending course of the air vehicle, during a substantiallyhorizontal flight of the air vehicle.

The program storage device may further embody instructions forcontrolling, during at least the second part of the descent, reductionof the groundspeed below a maximum permitted threshold speed before anend of the descent and before a beginning of the substantiallyhorizontal flight of the air vehicle.

The program storage device may further embody instructions forminimizing a vertical deviation of the air vehicle from a set altitudeconcurrently with at least a part of the controlling of the reducing ofthe groundspeed, and instructions for restricting the minimizing atleast by the restricting of the reduction of the thrust power based onthe lower threshold.

The program storage device may further embody instructions for furtherrestricting the minimizing, by restricting a rate of thrust powerreduction based on a maximal permitted thrust power reduction rate.

The program storage device may further embody instructions fordetermining the set groundspeed in response to a distance of the airvehicle from a predetermined hover destination position.

The program storage device may further embody instructions for repeatedchecking, starting at the first part of the descent and at least until afirst occurrence of an event selected from (a) receiving a negativeresult and (b) reduction of the groundspeed to substantial hovering, ifflight parameters are within an envelope permitting deceleration tohover at a predetermined hover destination position; and instructionsfor selectively instructing tilting of the at least one tiltablepropulsion unit to provide thrust in the general longitudinal thrustvector direction if a result of the checking was negative, andcontrolling a directing of the air vehicle to a position and a state inwhich the flight parameters are within the envelope, and reinitiatingthe method, starting again with the controlling during the first part ofthe descent.

The program storage device may further embody instructions fordetermining timing for tilting of the at least one tiltable propulsionunit between the first and the second parts of the descent forminimizing a duration between the tilting and the substantial hover.

The instructions for the controlling of the reducing of the groundspeedmay include instructions for restricting the reduction of thegroundspeed for preventing a reduction of the airspeed below apredetermined threshold until reaching a vicinity of a predeterminedhover destination position.

The program storage device may further embody instructions for keeping apitch of the air vehicle within a permitted pitch range concurrentlywith at least a part of the controlling of the reducing of thegroundspeed, wherein the permitted pitch range is dynamically determinedin response to the measured airspeed of the air vehicle.

The program storage device may further embody instructions fordetermining, prior to the controlling in the second part of the descent,a permitted air vehicle descending angle for at least a part of thedescending course based on atmospheric conditions, wherein thecontrolling of the descending course during at least a part of the firstpart of the descent is based on the permitted air vehicle descendingangle.

The program storage device may further embody instructions forcontrolling a substantially vertical descent of the air vehicle, afterreducing the groundspeed of the air vehicle substantially to hover,until a fulfillment of at least one ground detection condition.

The program storage device may embody instructions for controlling areduction of an altitude of the air vehicle during at least a part ofthe vertical descent, wherein the controlling of the reduction of thealtitude may include controlling modification of thrust power of the atleast one tiltable propulsion unit for minimizing a deviation of themonitored altitude of the air vehicle from a monotonically decreasingset altitude that decreases below a local ground level, more than fivetimes the height of the air vehicle.

The program storage device may further embody instructions forinitiating the controlling of the substantially vertical descent of theair vehicle if predetermined time elapsed from the tilting of the atleast one tiltable propulsion unit, regardless of sensors data.

The instructions for the controlling of the descending course mayinclude instructions for controlling an operation of at least oneaerodynamic part of the air vehicle selected from a group consisting ofan aileron, an elevator, a rudder, a ruddervator, a flaperon, elevons,and a wing flap.

FIG. 7 illustrates a possible exemplary flight course 1030 of airvehicle 100, during which it decelerates, according to an embodiment ofthe invention. Several sub-stages are exemplified in course 1030, whichmay correspond to stages of methods 1500 and 1600, and to control bycontrol system 1200.

In a first stage 1910, the descending process is engaged. Thedetermination of triggering the descending process may be takenautonomously by control system 1200 (e.g. due to a preplanned flightplan, due to remaining fuel level, or due to operational conditions suchas weather, etc.), and may also be determined by another system orperson. For example, control system 1200 may receive an instruction toland the air vehicle in a predetermined location, and possibly at apredetermined time. At the time of engaging the descending process, theairspeed of the air vehicle may be its cruising airspeed, or any speedat which it traveled before. If a descent is planned in advance, thisspeed may be lowered prior to engaging in the descent. In theillustrated example, the airspeed of the air vehicle when the descendingprocess is engaged is about 35 kts. The speed of 35 kts is merely anexample, as the actual velocity depends on many implementation factorssuch as the type of the air vehicle, its weight, the profile of thewing, environmental factors, etc.

Stage 1920 that follows includes a descending flight in the first flightmode, when the at least one tiltable propulsion unit 420 provides thrustin the general longitudinal thrust vector direction. While notnecessarily so, the non-tiltable propulsion unit 420 c (if implemented)does not provide any substantial thrust at this stage. The envelopepermitting descent to a substantial hover (illustrated by envelope 2000)is relatively large at this stage. Stage 1920 may or may not includedeceleration of the air vehicle, and may—in an example—include keeping asubstantially constant groundspeed of the air vehicle during that stage.The descending in stage 1920 may be achieved primarily by pitching theair vehicle down. The tendency to accelerate resulting from thedescending course (and the potential gravitational energy) may becompensated by slowing down the engines, creating drag, etc.

In stage 1930, the air vehicle switches to the second flightmode—essentially by tilting the at least one tiltable propulsion unitsto provide thrust in the general vertical thrust vector direction. Thethrust, previously used only for propelling the air vehicle (when liftis provided by the wing and potentially other surfaces), is directed inthis state to provide lift to the air vehicle, while also propelling it.Stage 1930 may also include activation of the previously inoperativenon-tiltable propulsion unit 420 c. In the illustrated example, thetilting is carried out at a height of some 70 m, and at a distance of500-600 m from the target. The location and/or timing of the tilting maybe determined based on energy consumption minimization considerations.The tilting may or may not include braking of rotating parts of thetiltable propulsion units. The distances and height provided above aremerely an example, and the actual values depend on many implementationfactors, such as the ones discussed above.

In stage 1940, the air vehicle continues to descend, now in the secondflight mode, when the at least one tiltable propulsion unit providesthrust in the general vertical thrust vector direction. It is noted thatpossibly, the course may include an intermediary stage between stage1920 and stage 1940, in which the at least one tiltable propulsion unitis directed in a diagonal tilt, providing thrust in an intermediarydirection between the general vertical thrust vector direction and thegeneral longitudinal thrust vector direction. Stage 1940 may or may notinclude deceleration of the air vehicle. For example, at an end of stage1940, the airspeed of the air vehicle may be reduced to about 90% of itsinitial value—e.g. to 32 kts. Other values may also be implemented (e.g.50-60%, 60-70%, 70-80%, about 85%, about 95%, etc.). The groundspeed maybe kept substantially constant during stage 1940, but this is notnecessarily so.

Stage 1950 includes slowing down the descent rate (possibly tosubstantially horizontal height). The slowing down of the descent ratemay enable more substantial slowing down, because less potentialgravitational energy is converted to kinetic energy each second.Therefore, stage 1950 may include a more significant slowing down thanthe preceding stages. For example, the airspeed of the air vehicle maybe reduced from the former 32 kts down to about 25 kts, a decrease ofabout 20%. Like before, the rate of airspeed decrease during stage 1950may be different in different implementations—ranging from zero and up,e.g. 5-10%, 10-20%, 20-30%, etc. In the illustrated example, stage 1950starts at a distance of some 200 from the hover destination location,and at an altitude of 10 m. The altitude in which stage 1950 starts maybe defined in relation to the ground (e.g. 10 m), but may also bedefined in multiplications of the horizontal flight altitude or thehover destination location hovering altitude.

In the illustrated example, the rate of slowing down is much higher thanthat of stage 1940. In stage 1940, the airspeed was reduced by 10% alongsome 350 m, while in stage 1950 the airspeed is slowed down twice asmuch—by about 20% —along a much shorter 100 m range. The length of stage1950 may be determined by geometrical considerations (e.g. distance fromthe hover destination location) as well as other considerations.

In stage 1960 the airspeed of the air vehicle is further reduced. Stage1960 may include substantially horizontal flight, but this is notnecessarily so. The rate of airspeed reduction may depend both onaerodynamic considerations (e.g. airspeed, stall angle, wind speed,etc), as well as on groundspeed considerations—the distance from thehover destination location, etc. The altitude at which the air vehicleflies in stage 1960 may be determined based on various factors—e.g. sizeof the air vehicle, geometry of the terrain, wind, altitude sensor errorrate, operational considerations, and so forth. Stage 1960 may continueuntil a final substantial stop at the hover destination location, but atit end stage 1965 of controlled rising of the air vehicle may beimplemented—e.g. for diminishing the residual groundspeed.

Potential stage 1970 may include hovering and keeping location (e.g.with respect to ground, to GPS data etc.). The duration of stage 1970may depend on various factors, such as time taking to sufficientlystabilize, operational considerations, etc.

In stage 1980 the air vehicle is lowered while keeping its location. Atsome point during the lowering, the ground detection sensors may betriggered, so that hitting the ground may be detected (if lowering to alanding). At an ultimate stage, when detecting landing on ground, theengines may be stopped.

The total time span from start of the descent process to substantialhover and possibly to the landing may last different durations indifferent implementations and under various conditions. The timinggreatly depends, for example, on size and weight of the air vehicle,aerodynamic capabilities thereof, power of the engines, weatherconditions, initial altitude of the air vehicle, course settingparameters, and so forth. By way of example only, for a 70 Kg unmannedair vehicle in the course exemplified with relation to FIG. 7, the totaltime span—from start to end—may be between 15 and 30 seconds.

FIG. 8 schematically illustrates control system 1200, according to anembodiment of the invention, as well as its environment. Control system1200 includes one or more input interfaces 1210, for receiving from oneor more external systems (e.g. sensors, detectors, controllers,navigation systems, etc.) information indicative of various parametersof air vehicle 100 and possibly of its environment. For example, suchinformation may pertain at least to the monitored airspeed of airvehicle 100 and to its monitored altitude. It should be noted that theindicative information does not necessarily include the value of therespective measured parameters, and such a value may also be deduced bycontrol unit 1200 based on information received from one or moresources.

A measured airspeed input interface 1210A may be implemented forreceiving information indicative of the measured airspeed of air vehicle100, from one or more airspeed detectors 490 (e.g. a Pitot tube). One ormore navigation system input interfaces 1210B may be implemented toreceive navigational information from a respective navigation systemsuch as GPS system 1310, from an inertial navigation system, from acamera based navigation system, and so forth. An altimeter inputinterface 1210C may be implemented to receive measured altitudeinformation from an altimeter 1320 mounted on the air vehicle. A thrustlevel indication input 1210D may be implemented to receive informationfrom one or more thrust level indicators 1330.

Other types of input interfaces may also be implemented, as will beclear to a person who is of skill in the art. It should be noted thatthe systems from which information is received are not necessarilymounted on the air vehicle, and are not necessarily sensors. Forexample, information may also be received from controllers of varioussystems of the air vehicle 100, providing status information regardingtheir status. Also, information may also be received from other aircraftand from ground stations. Furthermore, other inputs may be implementedin control system 1200 for receipt of instructions—e.g. destinationlocation, course setting instructions, landing instructions, and soforth.

Control unit 1220 may be implemented as a multitude of modules that mayor may not communicate with each other. Each of the modules may beimplemented on its own processor or by using dedicated hardware, butthis is not necessarily so. It is also noted that control unit 1220 maybe implemented as part of another control system of air vehicle 100,that is configured to control other aspects thereof (e.g. generalnavigation control system, a failure detection system, and so forth).

By way of example, some control modules are exemplified below. It willhowever be understood by a person who is of skill in the art that theimplementation of the disclosed control unit is not restricted to theexamples given. It is noted that the limitations and limiters discussedbelow may be implemented by hardware and/or by software. For example,limitation of thrust level may be implemented by actual restriction ofthrottle movement (in hardware) and/or by restriction of allowedparameters range (in software).

An altitude control module 1221 may be implemented for controlling thealtitude of air vehicle 100. This module may use information receivedfrom the altimeter input interface 1210C, and, based on a comparison ofthat measured altitude information with a set altitude, determine whatactions should be taken by one or more systems of air vehicle 100. Thealtitude control module 1221 may set the set altitude, but this altitudemay also be determined by a course setting module 1226. Some of themodules of the control unit 1220, such as course setting module 1226,may be modules that do not control systems of the air vehicle 100.

An airspeed control module 1222 may be implemented for controlling theairspeed of air vehicle 100. This module may use information receivedfrom an airspeed detector 490, and based on a comparison of the measuredairspeed with a set airspeed and/or based on differences betweenmeasured groundspeed to a set groundspeed, determine what actions shouldbe taken by one or more systems of air vehicle 100. The airspeed controlmodule 1222 may set the set airspeed, but this airspeed may also bedetermined by a course setting module 1226.

A groundspeed control module 1223 may be implemented for controlling thegroundspeed of air vehicle 100. This module may use information receivedfrom a groundspeed detector (e.g. an optical groundspeed detector,navigation system 1310, etc.), and, based on a comparison of themeasured groundspeed with a set groundspeed, and/or based on theairspeed of air vehicle 100, to determine what actions should be takenby one or more systems of air vehicle 100. The groundspeed controlmodule 1223 may set the set groundspeed, but this groundspeed may alsobe determined by a course setting module 1226. Airspeed control module1222 and groundspeed control module 1223 may also be implemented as asingle speed control module.

Control unit 1220 may include a speed control module (e.g. groundspeedcontrol module 1223 or a control module implemented as part of coursesetting module 1226) that is configured to determine the set groundspeedin response to a distance of the air vehicle from a predetermined hoverdestination position.

Other modules may also be implemented, such as (though not limited to)pitch, yaw, and roll control module 1224, navigation module 1225, energymanagement module, etc.

The control modules of control unit 1220 may interact with each other inorder to prevent contradictory instructions or other undesired effects.For example, if altitude control module 1221 determines that a pitch ofthe air vehicle 100 should be reduced as well as the thrust produced bythe tiltable propulsion unit 420 in the general vertical thrust vectordirection, then it may query the respective modules (e.g. queryingcontrol module 1224 regarding the pitch) if such an action can becarried out. An instruction to the tiltable propulsion unit 420 and/orto other components of the air vehicle 100 may be issued in such animplementation only if the carrying out of which is not blocked by anyof the participating control modules.

Control unit 1220 may include multiple limiters—either implemented aspart of the various control modules or externally to it, which are usedto limit the values of some parameters (e.g. pitch, throttle, etc.).These limiters may be implemented in software, in hardware, and/or infirmware.

The threshold values used for the limiting may be used to limit apotential exceeding of the air vehicle from permitted operational range,and may also be used for compensation of vehicle-specific response. Forexample, if the air vehicle 100 has a tendency to pitch up when airspeeddrops and thrust increases, the limiters (or other control forms) may beused to limit the pitching up and compensating for this tendency.

The threshold limiting values may be calculated by one or more modulesof the control unit 1220, but may also be gathered by those modules froma predefined database. For example, for any given airspeed, the pitch,yaw, and roll control module 1224 may query a look-up table (LUT), anduse values stored in the LUT for this airspeed as the higher and lowerpitch thresholds. Other control modules may just as well use look-uptables. Such LUTs may be stored, for example, in optional database 1230of control system 1200.

The values retrieved from the look-up table (or other database) may beused as is, but may also serve as a basis for a change. Continuing thesame example, control module 1224 may retrieve a nominal pitch value(e.g. used for correction and/or compensation) to which the pitch of theair vehicle should preferably be modified, and based on that nominalpitch value, to implement a control loop and then to correct the pitchaccordingly. Such correction of nominal values derived from the one ormore LUTs by the different control modules may be based on values ofother parameters, and restricted using thresholds (e.g. as shown inFIGS. 5 and 6).

Some of the controllers to which control unit 1220 and its differentcontrol modules may issue commands are tiltable propulsion unit control1410 controlling one or more of the at least one tiltable propulsionunit 420; non-tiltable propulsion unit control 1420 controlling one ormore of the possibly implemented non-tiltable propulsion units; anaileron controller 346 for controlling one or more ailerons, ifimplemented; a rudder controller 1440 for controlling one or morerudders, if implemented. A ground detection sensors controller 1450 maybe implemented for selectively activating the one or more grounddetection sensors—e.g. at a predetermined altitude.

In an example, control unit 1220 (and different components thereof) maybe configured to issue the controlling commands to the controllers ofthe aerodynamic subsystems for controlling an operation of at least oneaerodynamic subsystem of the air vehicle selected from a groupconsisting of an aileron, an elevator, a rudder, a ruddervator, aflaperon, elevons, and a wing flap.

It should be noted that while not necessarily so, control unit 1200 maybe configured to control the descending course of air vehicle 100 andthe reducing of the groundspeed thereof automatically, and/orautonomously—without any external intervention.

It is noted that air vehicle 100 may include one or more wings 320. Thewings and the propulsion units may have contradictory effects (e.g.while attempting to reduce lift by reducing a rotation speed of a rotorof the tiltable propulsion unit tiltable propulsion unit 420, the wingmay increase its generated lift due to a change in pitch).

Changes in the angle of attack that increase the lift generated by thewind naturally reduces the relative lift which is ought to be providedby the engines to provide a constant overall lift. Over a certain angle,the lift provided by the wing is small and possibly even negligible, andchanging the pitch of the air vehicle above that angle would result inmodifying the flight direction only as depending on the power of theengines and the direction of the thrust. Thus, climbing of the airvehicle is enabled in such case without changing the pitching.

The transit between the first state in which the pitching angle affectsthe climbing of the air vehicle to the second state in which only therotors affect the climbing is a gradual transition which occurs as partof the slowing down process (though not only then). During slowing downsuch a transition may start around the stalling velocity of the airvehicle, and the transition to that second state is concluded when themost of the weight of the air vehicle is supported by the power of therotors. In this state, the control of the altitude of the air vehicle isdone by controlling the engines.

All the more so, thrust airflow thrust by the tiltable propulsion unit420 may be thrust onto the wing and modify the airflow around it, orhave other aerodynamic effects on various aerodynamic subsystems of airvehicle 100. Control unit 1200 may be configured to balance betweencontradictory aerodynamic effects resulting from the wing and from theat least one tiltable propulsion unit.

As mentioned in relation to method 1500, part of the course of airvehicle 100 may be horizontal. The control unit 1200 may be configuredto control the reducing of the groundspeed of the air vehicle during asubstantially horizontal flight of the air vehicle (e.g. at least partlyafter the ceasing to control the descending course of the air vehicle).

As mentioned in relation to method 1500, the controlling of the airvehicle may include controlling a slowing down thereof during itsdescent. Control unit 1220 may be configured to control, during at leastthe second part of the descent, reduction of the groundspeed below amaximum permitted threshold speed before an end of the descent andbefore a beginning of the substantially horizontal flight of air vehicle100.

As mentioned in relation to method 1500, in some implementations thekeeping of the thrust levels within permitted range may over-ride and beprioritized over keeping of the air vehicle at a fixed height and/or atwithin its set altitude permitted range. Control unit 1200 may includealtitude control module 1221 which in turn may be configured to minimizea vertical deviation of air vehicle 100 from a set altitude when thecontrol unit 1200 controls the reducing of the groundspeed, and torestrict the minimizing at least based on the restricting of thereduction of the thrust power based on the lower threshold.

As mentioned in relation to method 1500, in some implementations thekeeping of the rate of the reduction of may over-ride and be prioritizedover the keeping of the air vehicle at a fixed height and/or at withinits set altitude permitted range. The altitude control 1221 module maybe configured to further restrict the minimizing by restricting a rateof thrust power reduction based on a maximal permitted thrust powerreduction rate.

Either control unit 1200, another system of air vehicle 100, a pilotthereof, or a remote operator or a remote control system may be able todetermine when to cancel the descent and/or slowing down to a hoverprocess. For example, control unit 1200 may further include a monitor orother control module (not illustrated separately), configured torepeatedly check, starting at the first part of the descent and at leastuntil a first occurrence of an event selected from (a) receiving anegative result and (b) reduction of the groundspeed to substantiallyhovering, whether flight parameters are within an envelope permittingdeceleration to hover at a predetermined hover destination position;wherein control unit 1200 is configured to selectively instruct tiltingof the at least one tiltable propulsion unit 420 to provide thrust inthe general longitudinal thrust vector direction if a result of thechecking was negative, and to further control a directing of the airvehicle 100 to a position and a state in which the flight parameters arewithin the envelope. A process like the one disclosed in relation tomethod 1500 may be reinitiated, starting again with the controllingduring the first part of the descent, but this is not necessarily so.

As aforementioned, the energy consumption of air vehicle 100 when in thesecond flight mode is substantially higher than when in the first flightmode. Control unit may therefore further include a tilting controlmodule (not illustrated as a standalone module) that is configured todetermine timing for tilting of the at least one tiltable propulsionunit 420 between the first and the second parts of the descent, forminimizing a duration between the tilting and the substantial hover.

Since premature slowing down of the air vehicle may be hard to overcome,control unit 1200 may be configured to restrict the reduction of thegroundspeed for preventing a reduction of the airspeed below apredetermined threshold until reaching a vicinity of a predeterminedhover destination position.

Referring to FIGS. 6 and 8, control unit 1200 may include a pitchcontrol module (may be implemented as part of control module 1224) thatis configured to keep a pitch of the air vehicle within a permittedpitch range when the control unit controls 1200 the reducing of thegroundspeed (but not necessarily during all of the groundspeedreduction), wherein the permitted pitch range is dynamically determinedin response to the measured airspeed of the air vehicle 100. That pitchcontrol module may be configured to prevent stalling of the air vehicleat least by keeping the pitch within the permitted pitch range

The access angle of air vehicle 100 may be controlled in various ways.For example, control unit 1220 may be configured to: (a) determine,before the second part of the descent (possibly even before the firstpart) a permitted air vehicle descending angle for at least a part ofthe descending course based on atmospheric conditions, and (b) controlthe descending course during at least a part of the first part of thedescent based on the permitted air vehicle descending angle.

While not necessarily so, after the air vehicle is slowed down tosubstantially a hover, it may controllably be lowered down substantiallyvertically—e.g. for a landing of air vehicle 100 or to another hoveringposition. Control unit 1220 may be configured to control a substantiallyvertical descent of the air vehicle, after the reducing of thegroundspeed of the air vehicle substantially to hover, until afulfillment of at least one ground detection condition (or othercondition, if being lowered to a hovering rather than to a landing).

The techniques detailed with relation to method 1600 may be implemented,as aforementioned, also with respect to control system 1200, andespecially with respect to control unit 1220. For example, control unit1220 may include a landing altitude control module 1227 that isconfigured to control a reduction of an altitude of the air vehicle 100during at least a part of the vertical descent, and to controlmodification of thrust power of the at least one tiltable propulsionunit 420 when controlling the reduction of the altitude, for minimizinga deviation of the monitored altitude of the air vehicle 100 from amonotonically decreasing set altitude that decreases below a localground level more than five times a height of the air vehicle. Thevariations discussed with relation to method 1600 (and especially withrelation to stage 1620 thereof) may also be implemented with relation tosystem 1200 and to control unit 1220.

Control unit may further be configured to initiate the controlling ofthe substantially vertical descent of the air vehicle if predeterminedtime elapsed from the tilting of the at least one tiltable propulsionunit, regardless of sensors data.

FIG. 9 schematically illustrates control system 1201, according to anembodiment of the invention, as well as its environment. Control system1201 may be used for controlling flight of air vehicle 100 in some orall of its stages. For example, control system 1201 may be used forcontrolling a deceleration process, an acceleration process, a loweringprocess, a climbing process, a landing process, a take-off process, ahover process, etc. Especially, control system 1201 may implement method1500 and/or method 3000.

Control system 1201 may be fully automated and autonomous, but in someimplementations it may also react to commands issued by another one ormore systems and/or persons. For example, if over-ridden by a humanissued command, control system 1201 may stop its autonomous control ofthe air vehicle 100, which is then controlled by another system or bythe issuing person (either by mediation of control system 1201 orotherwise).

Optionally, control system 1201 may implement some or all of thefunctionalities discussed with respect to control system 1200. Forconvenience of explanation, some components of system 1201 haveidentical reference numerals. Each of these components when discussedwith respect to system 1201 may be implemented in a similar manner tothe one discussed with respect to system 1200, but this is notnecessarily so. Especially, components whose reference number includes adecimal point may optionally be implemented as a part of a component ofthe same reference number—without the fraction part (i.e. rounded downto the integer).

Optionally, control system 1201 may include process management module1240. Process management module 1240 may be used to select betweendifferent control schemes by which other components of system 1201operate. For example, in different control schemes, controlling of thealtitude of air vehicle 100 may be governed by a climb rate (RoC—rate ofclimb) command, and/or by an elevation command. Process managementmodule 1240 may be configured to instruct other modules of system 1201which types of control commands to issue/follow, and according to whichdecision rules.

For example, process management module 1240 may be configured to applydifferent control schemes for other components of system 1201 indifferent stages of a process such as: deceleration process,acceleration process, lowering process, climbing process, landingprocess, take-off process, etc. Especially, different control schemesmay be used when the one or more tiltable propulsion units 420 providesthrust in the general longitudinal thrust vector direction. One suchcontrol scheme, for example, may be based on RoC input and correspondingRoC commands, while another scheme may be based on an altitude sensorinput and corresponding altitude commands. As discussed below in greaterdetail, process management module 1240 may determine at which out ofseveral distinguished stages of an acceleration and/or ascending processthe system is at, and consequentially—which of the control loops (orother types of control schemes) should be used.

Even though control system 1201 may be used to control an air vehicle indifferent situations, it especially may be configured to control anacceleration of an air vehicle (such as air vehicle 100) which includesa tiltable propulsion unit (e.g. unit 420) that is tiltable to provide athrust whose direction is variable at least between a general verticalthrust vector direction and a general longitudinal thrust vectordirection with respect to the air vehicle.

Very generally, input data of various sensors is provided to a controlunit 1290 (which may include various modules), which in turn control amodifying of the state of various controls of the air vehicle (e.g.tiltable propulsion unit, elevator, flaps, etc.), a modifying which isexecuted with the help of various respective controllers (collectivelydenoted 1400). The modification in the state of these controls resultsin changed physical conditions which in turn affect the data sensed bythe sensor and therefore their output. Such new output is again fed tothe control unit 1290, and this cycle continues. It is noted that someor all of the controllers (collectively denoted 1400) of the air vehiclecontrols may be part of system 1201, but not necessarily so. Likewise,some or all of the sensors (collectively denoted 1300) of the airvehicle controls may be part of system 1201, but not necessarily so.Controllers 1400 may be a part of system 1201, but not necessarily so.

It is noted that while some of the examples illustrated and/or discussedrefer to classical control method (proportional-integral-derivativecontrollers, PID controllers), other control methods may be implemented,such as but not limited to close-loop or open loop control method,including robust control methods, optimal control methods, non-linearmethods, etc.

The ways in which system 1201 may operate may become still clearer whenviewed in light of method 3000, and of the processes described withrespect to FIGS. 10, 14, 15, 16, 17, 18, and 19.

Input interface may be configured to receive data from various sensors,selected from a wide group of sensors which includes (but is not limitedto): sensors indicating aerodynamic conditions of the air vehicle,sensors indicating meteorological conditions, sensors indicating statesof air vehicle controls, and so on. Especially, input interface 1210 isconfigured to receive information indicative of a monitored airspeed ofthe air vehicle (e.g., from airspeed detector 490).

Control unit 1290 is configured to issue controlling commands to one ormore controllers of one or more air vehicle controllers. Especially,control unit 1290 is configured to issue controlling commands to one ormore controllers of one or more air vehicle controllers for controllingthe acceleration of the air vehicle. While different accelerationprocesses may be controlled by control unit 1290, an accelerationprocess which may be controlled by control unit 1290 is an accelerationprocess in which the airspeed of the air vehicle is increased by atleast 10 knots and at least by a ratio of 1.5. Control unit 1290 mayissue control commands to various types of controllers, and especially,control unit 1290 is configured to issue controlling commands to acontroller 1410 of the tiltable propulsion unit tiltable propulsion unitfor controlling the acceleration of the air vehicle.

The controlling of the acceleration by control unit 1290 may be dividedinto two parts. In the first part of the acceleration the tiltablepropulsion unit provides thrust in the general vertical thrust vectordirection and in the second part of the acceleration the tiltablepropulsion unit provides thrust in the longitudinal thrust vectordirection. Control unit 1290 is configured to control an operation ofthe at least one tiltable propulsion unit (and possibly of othercomponents of the air vehicle as well) during both of these parts of theacceleration.

Generally, in the first part of the acceleration (in which the tiltablepropulsion unit provides thrust in the general vertical thrust vectordirection), the control unit 1290 is configured to: (a) control anoperation of the at least one tiltable propulsion unit for providinglift to the air vehicle, and (b) control a modifying of a tilt angle ofthe tiltable propulsion unit with respect to a fuselage of the airvehicle, based on: (i) the monitored airspeed and (ii) an airspeedcommand.

Following a tilting of the at least one tiltable propulsion unit (whichmay be triggered and controlled by control unit 1290), control unit 1290is configured to control in the second part of the acceleration anoperation of the tiltable propulsion unit to provide thrust in thegeneral longitudinal thrust vector direction for propelling the airvehicle.

It is noted that the controlling of the modifying of the tilt angle atthe first part (and possibly also in other parts) of the acceleration isbased on the monitored airspeed as such, and not exclusively (if at all)on derivative parameters such as airspeed error (which is the differencebetween a momentary monitored airspeed and a planned airspeed for thatmoment). Especially, optionally control unit 1290 may be configured tocontrol the modifying of the tilt angle based on the monitored airspeedindependently of the airspeed error.

The controlling of the tilt angle (at least in the first part) is alsobased on the airspeed command. The airspeed command itself may be basedon a preplanned acceleration function. For example, the airspeed commandmay instruct for a moment T airspeed V so that V=f(T) (i.e. irrespectiveof other factors). For example, the airspeed command may instructairspeed V=V₀+a×T.

As an optional way of being based on the airspeed command, control unit1290 may be configured to control the tilt angle (at least in the firstpart of the accelerating) based on a parameter which is in turn based onthe airspeed command (i.e., derived therefrom), such as an airspeederror (an error of the monitored airspeed with respect to the airspeedcommand). That is, optionally the controlling of the modifying of thetilt by control unit 1290 may be based on (i) the monitored airspeed;and (ii) the airspeed error.

Control system 1201 may be fully automated and autonomous, but in someimplementations it may also react to commands issued by another one ormore systems and/or persons. For example, if over-ridden by a humanissued command, control system 1201 may stop its autonomous control ofthe air vehicle 100, which is then controlled by another system or bythe issuing person (either by mediation of control system 1201 orotherwise). Optionally, control unit 1290 may be configured to controlthe operation of the tiltable propulsion unit automatically.

It is noted that control unit 1290 may include many modules, each ofwhich is in charge of one or more aspects of the controlling of airvehicle 100. While not necessarily so, the modules of control unit 1290may include at least two main types of modules:

-   -   Navigational parameters management modules (collectively denoted        1250), also referred to as “external loop modules”; and    -   Aerodynamic parameters management modules (collectively denoted        1260), also referred to as “internal loop modules”.

The navigational parameters management modules manage where the airvehicle should be and at what speed, and issue respective controlcommands to the respective aerodynamic parameters management modules,which manage the required aerodynamic response which would bring the airvehicle towards the required location and position. The aerodynamicparameters management modules then issue control commands which instructthe modifying of the state of one or more controllers of the air vehiclein a way which would bring the air vehicle response towards the requiredaerodynamic response.

Navigational parameters management modules 1250 may include, forexample: some or all of the following:

-   -   Altitude management module 1221.1, which is configured to issue        an altitude control command, indicating at which altitude the        air vehicle should be at the present (or a predefined future        time). Module 1221.1 may issue the altitude command based on a        predefined altitude profile, based on information received from        sensors (e.g., ground elevation, measured altitude), and so on.    -   Climb rate (RoC) management module 1221.2, which is configured        to issue a RoC control command, indicating the rate at which the        air vehicle should ascend/descend at the present (or a        predefined future time). Module 1221.2 may issue the RoC command        based on a predefined altitude profile, based on information        received from sensors (e.g., ground elevation, measured        altitude, measured RoC), and so on.    -   Longitude airspeed management module 1222.1, which is configured        to issue a longitudinal velocity control command, indicating at        which longitudinal velocity the air vehicle should fly at the        present (or a predefined future time). Module 1222.1 may issue        the longitudinal velocity command based on a predefined        flight/velocity profile, based on information received from        sensors (e.g., ground distance covered, wind speed, measured        airspeed), and so on.    -   Sideways airspeed management module 1222.2, which is configured        to issue a sideways velocity control command, indicating at        which longitudinal velocity the air vehicle should fly at the        present (or a predefined future time). Module 1222.2 may issue        the sideways velocity command based on a predefined        flight/velocity profile, based on information received from        sensors (e.g., ground distance covered, wind speed, measured        airspeed), and so on.    -   Groundspeed management module 1223, which is configured to issue        a groundspeed control command, indicating at which velocity with        respect to the local ground the air vehicle should fly at the        present (or a predefined future time). Module 1223 may issue the        groundspeed command based on a predefined flight profile, based        on information received from sensors (e.g., ground distance        covered, GPS positioning data, measured groundspeed), and so on.    -   Location management module 1225.1, which is configured to issue        a location control command, indicating at which location the air        vehicle should be at the present (or a predefined future time).        Module 1225.1 may issue the location command based on a        predefined flight course, based on information received from        sensors (e.g., based on processing of camera's video, GPS        positioning data, measured groundspeed), and so on.    -   Heading management module 1225.2, which is configured to issue a        heading control command, indicating at which direction the air        vehicle should fly at the present (or a predefined future time).        Module 1225.2 may issue the heading command based on a        predefined flight course, based on information received from        sensors (e.g., based on processing of camera's video GPS        positioning data, measured groundspeed, measured wind), and so        on.

It is noted that other navigational parameters management modules mayalso be implemented. Also, some pairs of these modules provide controlcommands which are somewhat overlapping in scope. For example,determining of the thrust level required for changing the elevation ofthe air vehicle may be determined based on the altitude command and on aRoC command, but usually there is no need for two such commands to beissued in parallel. Especially, given knowledge of a present altitude ofthe air vehicle and an altitude command, the required RoC may be derivedfrom such knowledge.

Therefore, process management module 1240 may determine which of modules1221.1 and 1222.2 would issue commands at any given control scheme. Theother module out of these two modules may be inactive at the time.Alternatively, one of these modules (e.g. RoC management module 1221.2)may issue the control commands on which the respective one or moreaerodynamic parameters management modules base their operation, whilethe other module (e.g. altitude management module 1221.1) may only setthresholds which ought not to be crossed. For example, in one controlscheme such respective one or more aerodynamic parameters managementmodules may base their operation on the RoC command, while the altitudedoes not fall below a lower altitude threshold. If such a lowerthreshold is crossed, they switch to basing their operation on thealtitude command, until the air vehicle ascends above that threshold.

Each of the modules 1250 may operate independently of the operation ofother modules (e.g. by implementing decision rules which depend only oninformation received from sensors, but not from modules of the externalloop level). Alternatively, at least one of the modules 1250 may operatebased on information determined by one of the other modules 1250.

This may be done hierarchically or in cooperation. In a hierarchicaloperation, one module depends on the result of another module, but notthe other way around, e.g., the heading management module 1225.2, mayissue a heading command, and following that the sideways airspeedmanagement module 1222.2 may issue its control command further based onthe tilt command. In cooperation, both modules would issue therespective control commands based on intermediate information receivedfrom one another (in a sort of dialogue). Is such cases, two suchmodules may be implemented together, using at least partly commondecision rules.

Aerodynamic parameters management modules 1260 may include, for example:some or all of the following:

-   -   Pitch control module 1224.1, which is configured to issue a        pitch command based on one or more control commands received        from at least one of the navigational parameters management        modules 1250 (e.g. altitude command, airspeed command) and on        information received from sensors (e.g. measured pitch, measured        airspeed, etc.).    -   Roll management module 1224.2, which is configured to issue a        roll command based on one or more control commands received from        at least one of the navigational parameters management modules        1250 (e.g. altitude command, airspeed command) and on        information received from sensors (e.g. measured roll, measured        airspeed, etc.).    -   Yaw management module 1224.3, which is configured to issue a yaw        command based on one or more control commands received from at        least one of the navigational parameters management modules 1250        (e.g. heading command, location command) and on information        received from sensors (e.g. measured yaw, measured airspeed,        etc.).    -   Throttle management module 1261, which is configured to issue a        throttle command for one or more of the at least one tiltable        propulsion units, based on one or more control commands received        from at least one of the navigational parameters management        modules 1250 (e.g. altitude command, RoC command, pitch command,        etc.) and on information received from sensors (e.g. current        throttle stage, measured pitch, etc.).    -   Tiltable propulsion unit plane management module 1262, which is        configured to issue a tilt command for one or more of the at        least one tiltable propulsion units, based on one or more        control commands received from at least one of the navigational        parameters management modules 1250 (e.g. altitude command, RoC        command, pitch command, etc.) and on information received from        sensors (e.g. current throttle stage, measured pitch, etc.).

It is noted that group 1260 may include management modules which manageparameters which may be regarded as more than strictly aerodynamic(e.g., throttle of the tiltable propulsion unit). However, theaerodynamic response of the air vehicle depends on such modules and istherefore managed at the same level of control modules.

For example, the lift which is required to satisfy an altitude commandissued by altitude management module 1221.1 is affected by the pitch andthe roll of the air vehicle (possibly also by the yaw), but also by thetilt angle and the throttle (which affect the thrust generated by thetiltable propulsion unit).

Each of the modules 1260 may operate independently of the operation ofother modules (e.g. by implementing decision rules which depend only oninformation received from sensors and from the external loop level, butnot from modules of the internal loop level). Alternatively, at leastone of the modules 1260 may operate based on information determined byone of the other modules 1260.

This may be done hierarchically or in cooperation. In a hierarchicaloperation, one module depends on the result of another module, but notthe other way around, e.g., pitch control module 1224.1 may issue apitch command, and following that the tiltable propulsion unit planemanagement module 1262 would issue the tilt command further based on thetilt command. In cooperation, both modules would issue the respectivecontrol commands based on intermediate information received from oneanother (in a sort of dialogue). Is such cases, two such modules may beimplemented together, using at least partly common decision rules.

For at least some of the commands of the external loop, different setsof aerodynamic parameters may reach the desired results in differentways. For example, the RoC command may be fulfilled by different ratiosbetween the lift generated by the wings (depending primarily on thepitch and the airspeed) and the lift resulting from the thrust generatedby the at least one tiltable propulsion unit (depending primarily onpitch, tilt and throttle). The ratio eventually selected (explicitly orimplicitly) may be determined in an independent manner, in ahierarchical manner or in a cooperative manner, as discussed above.

Optionally, control unit 1290 may be configured to change a ratio(either explicitly determining it or only implicitly changing it)between: (a) a contribution of the tiltable propulsion unit to themodification of the pitch and (b) a contribution of at least oneelevator (or other kinds of aerodynamics controls other than thetiltable propulsion unit) to the modification of the pitch, wherein thechanging of the ratio is correlated (optionally:monotonously) to themonitored airspeed.

As aforementioned with respect to air vehicle 100, the air vehicle mayinclude a wide variety of controls (also referred to as aerodynamicsubsystems, collectively denoted 4000). These aerodynamic subsystems mayinclude any combination of: the at least one tiltable propulsion unit,at least one non-tiltable propulsion unit, a throttle, an engine,ailerons, elevators, rudder, ruddervator, flaperons, elevons, wingflaps, slats, spoilers, air brakes, variable-sweep wings, blades ofrotors, cyclic, collective, and so on.

Control unit 1290 may control an operation of at least one aerodynamicpart of the air vehicle (100) selected from a group which includes anaileron, an elevator, a rudder, a ruddervator, a flaperon, elevons, anda wing flap.

Each out of the aerodynamic subsystems of the air vehicle may becontrolled by a matching controller of the group of controllers 1400.However, optionally one or more of these controllers may be configuredto control multiple aerodynamic subsystems. Furthermore, optionally airvehicle 100 may include some aerodynamic subsystems (or at leastsubsystems whose activation may have aerodynamic affects, e.g. landinggear which may be extracted or contracted) which are not controlled bysystem 1201.

The controllers 1400 may include, for example, some or all of thefollowing:

-   -   Tiltable propulsion unit controller 1410 configured to control        one or more of the at least one tiltable propulsion unit        tiltable propulsion units 420.    -   Non-tiltable propulsion unit control 1420 configured to control        one or more of the possibly implemented non-tiltable propulsion        units.    -   Aileron controller 346 configured to control one or more        ailerons, if implemented.    -   Rudder controller 1440 configured to control one or more        rudders, if implemented.    -   RPM control    -   Main rotor(s) common blade pitch angle control (“collective”)    -   Main rotor(s) cyclic blade pitch angle control (“cyclic”)    -   Tail rotor(s) common and cyclic blade pitch angle control    -   Others

As indicated above, the various aerodynamic parameters management 1260issue control commands which instruct the modifying of the state of oneor more controllers of the air vehicle in a way which would bring theair vehicle response towards the required aerodynamic response (whichwould bring the air vehicle towards the required location and positionindicated by the respective navigational parameters management modules1250).

For at least some of the commands of the internal loop (indicatingaerodynamic parameters such as pitch), different sets of responses ofthe air vehicle's controls may reach the desired results in differentways. For example, the pitch command may be fulfilled by differentratios between aerodynamic effects of the tiltable propulsion unit, ofthe flaps, of the elevators, etc. The ratio eventually selected(explicitly or implicitly) may be determined by the respective internalloop control module (e.g. by pitch control module 1224.1 in the case ofpitch), or by the lower level of controllers 1400.

Clearly, in the latter case the modifications of the various controlsmay be determined in the controllers 1400 in an independent manner, in ahierarchical manner or in a cooperative manner, as discussed above.However, also in the former case, the ratio between the differentcontrols may be affected by the control modules of the internal loop inan independent manner, in a hierarchical manner or in a cooperativemanner, as discussed above. For example, control of the flaps foraffecting the roll may also affect the pitch, and should be accountedfor.

It is noted that control system 1201 may implement various controlchannels which have little relations (if any) in the external looplevel, and only affect each other in the lower levels (especially ofcontrollers 1400).

Some such control channels are:

-   -   Airspeed command→Airspeed loop→Pitch command→Pitch        loop→Effectors (Motors, elevator, etc. . . . )    -   Z (altitude) command→dZ loop→Rate of climb command→Rate of climb        loop→throttle command→Effectors (Motors)    -   Tilt angle command→tilt rate limiter→effectors (tilt servo's)

Longitude airspeed management module 1222.1 issues a longitudinalvelocity control command, indicating at which longitudinal velocity theair vehicle should fly at the present (or a predefined future time).Module 1222.1 (also referred to as “the airspeed command module”, whichmay also include module 1222.2) sets the value of required airspeedduring the acceleration phase.

Optionally, the airspeed command may start at t=0 (when the accelerationprocess initiates, possibly only after a preliminary takeoff/ascendprocess) with the same value as the measured airspeed. It is noted thatthe measured airspeed is not necessarily zero—there may be a residualairspeed from a previous flight of the air vehicle, and even asubstantially vertical ascent in takeoff may lead to a non-zero measuredairspeed due to wind. Continuing the same example, at times t>0, thelongitudinal airspeed command gradually and linearly increases with timeat a fixed rate (e.g. 2.0 kt/sec). The airspeed command may be increasedindefinitely in the same rate, or the rate may be changed (e.g. afterreaching a threshold, optionally predetermined before the acceleration,such as 15 knots).

FIG. 13 is a graph 5100 illustrating an airspeed command (line 5110)that may be issued by Longitude airspeed management module 1222.1,according to an embodiment of the invention. Also illustrated is anexemplary airspeed error which may develop (line 5120). As illustrated,the rate changes after the airspeed threshold to a lower rate (e.g.,only 1.5 kt/sec). This facilitates (and possibly enables) keeping thepitch angle of the air vehicle positive and gains positive lift from thewings.

Additionally, the airspeed command module may implement a “commandfreeze” mechanism: if the airspeed error exceeds a predetermined level(in the illustrated example, 5 knots), the command freezes. Ifimplemented, this feature enables avoiding reaching a pitch commandlower limiter and enables the airspeed loop to correct the pitch commandin both directions, gaining better cope with winds, gusts and severeweather conditions. Internal integrators may be implemented in theairspeed and inner pitch loops (illustrated in FIG. 14), therebyensuring zero steady state error, hence the freeze in the command modulewill not stop the aircraft from accelerating.

FIG. 14 illustrates interrelations between various components of system1201, according to an embodiment of the invention.

Pitch control module 1224.1 receives a speed command (an airspeedcommand or a groundspeed command) and a respective measured speed (i.e.,measured airspeed or measured groundspeed, respectively). In theillustrated embodiment, either airspeed or groundspeed is managed ateach time, based on the flight mode selection of process managementmodule 1240. For example, during substantially vertical takeoff,groundspeed may be managed, while during flight forward acceleration,airspeed may be managed. However, both airspeed and groundspeed mayoptionally be managed concurrently (e.g., in a transition stage betweentakeoff and forward acceleration). In other implementations, groundspeedis not necessarily an optional input for pitch control module 1224.1.

A difference between the measured speed and the speed command iscalculated by speed loop 1224.11 (which is implemented as part of pitchcontrol module 1224.1, but may also be implemented as part of therespective speed management module, e.g., module 1222.1). While thepitch command may be based on the speed error and on the measuredairspeed, it may also be based on the altitude error, as exemplified bythe altitude error input received from the altitude loop (which may be apart of Altitude (dz) management module 1221.1 or from pitch controlmodule 1224.1. It is noted that RoC error may be implemented instead ofaltitude error. In the given example, the altitude error and the speederror are summed (i.e., the pitch command has positive linear relationto the speed error, and a positive linear relation to the altitudeerror). In the illustrated example, the pitch command is determinedfurther based on the measured airspeed in that the limiters for thepitch command are responsive to the measured airspeed.

It is noted that the output of the speed loop 1224.11 may also betransferred to tiltable propulsion unit plane management module 1262 fordetermining the tilt angle command Optionally, peed loop 1224.11 andoptionally pitch control module 1224.1 may be implemented usingclassical PI (Proportional-Integral) control laws.

The reaction of the air vehicle may therefore be implemented (e.g.according to the control scheme illustrated in FIG. 14) as:

-   -   If the speed is too low, the nose of the air vehicle is pitched        down, and vice versa.    -   If the altitude is too low, the nose is pitched up, and vice        versa.    -   Pitch limiters are based on the measured airspeed, and        correspond to a stall envelope of the air vehicle (such limiters        keep the pitch within a range of attach angles in which the air        vehicle does not stall).

Pitch control module 1224.1 is configured to issue the pitch commandbased on the monitored airspeed and on the airspeed command, andpossibly further in response to the measured altitude of the air vehicle(e.g. as discussed with reference to FIG. 14). Especially, Pitch controlmodule 1224.1 may be configured to issue the pitch command based on themonitored airspeed, on the airspeed command, and on an altitude error.

Pitch control module 1224.1 (also referred to as “pitch command module”)is configured to issue a pitch command based on one or more controlcommands received from at least one of the navigational parametersmanagement modules 1250 (e.g. altitude command, airspeed command) and oninformation received from sensors (e.g. measured pitch, measuredairspeed, etc.).

Pitch control module may be configured to combine the speed loop outputand an external pitch command bias (e.g., of −10°) relative to hoverpitch, which connected at t=0 (upon switching to “accelerate” mode). Thegoal of this bias is to assist the speed loop at the beginning of theacceleration and to shorten the overall time of acceleration (energyconservation).

Optionally, pitch control module 1224.1 may be configured to keep apitch of the air vehicle (100) during at least a part of the first partof the acceleration within a permitted pitch range that is dynamicallydetermined in response to the measured airspeed of the air vehicle (100)

FIG. 15 illustrates interrelations between various components of system1201, according to an embodiment of the invention.

Optionally, throttle management module 1261 may be configured to sum theRoC command from the rate-of-climb module with a pre-defined throttlebias value which is a function of the airspeed (from throttle biasmodule 1261.2, e.g., in look-up-table form). The bias of throttle bias1261.2, if implemented, compensates for non-linearity of the air vehicle(and especially of a throttle response thereof in different airspeeds),and might be omitted if a non-linear control method is implemented.

Altitude management module 1221.1 sets the desired altitude during theacceleration phase. The altitude can be constant (flight path angleγ=0°) or variable (γ>0 i.e. ascend, γ<0 i.e. descend). The desiredaltitude may be calculated based on the nominal altitude command and theflight path angle—determined with respect to the location of the airvehicle on t=0 (upon switching to “accelerate” mode). Optionally,altitude management module may implement a Dz loop module 1221.11 whoseoutput “dZ” is the altitude error (desired altitude minus the measuredaltitude of the air vehicle).

Optionally, control unit 1290 (e.g., module 1261) may be configured tocontrol thrust power of the tiltable propulsion unit for reducing adifference between the measured airspeed and a set airspeed, whilerestricting increasing of the thrust power based on a threshold (eitherof the one or more thresholds of limiter 1261.1) that is determined inresponse to the measured airspeed.

The thrust thresholds may override the airspeed command, that is,control unit 1290 (e.g., throttle management module 1261) may beconfigured to control thrust power of the tiltable propulsion unit forreducing a difference between the measured airspeed and a set airspeed,while restricting reduction of the thrust power based on a lowerthreshold (either of the one or more thresholds of limiter 1261.1) thatis determined in response to a measured airspeed of the air vehicle.

Staying within the permitted thrust/throttle range (defined by thethresholds) may override keeping fixed/predefined height. That is,altitude control module (1221.1) may be configured to minimize avertical deviation of the air vehicle from a set altitude during thefirst part of the acceleration, wherein the minimizing is restricted atleast by the restricting of the reduction of the thrust power based onthe lower threshold.

Optionally, the altitude of the air vehicle may be controlled based onthe RoC command (of module 1221.1), with the exception that one or morealtitude limiters may be implemented (especially a lower one) tooverride the RoC based control scheme if crossed. The altitude loopcontrols the air vehicle altitude, minimizing dZ (altitude error).During the acceleration, the air vehicle heave axis may be controlled bythe inner rate-of-climb loop. Optionally, the dZ loop only connects inone (positive—climb up) direction and only if for some reason the airvehicle crossed the desired flight path towards the ground (downwards)by more than a predefined threshold (e.g. 5 meters).

The output of the dZ loop (which is proportional to the error) is summedto the nominal rate-of-climb command to compensate for rate-of-climbsensor errors, severe weather conditions, etc. designed to increaseflight safety during takeoff and to avoid controlled flight into terrain(CFIT).

FIG. 16 is a graph 5200 illustrating the measured altitude, the altitudecommand, the difference between them (dZ), and the issued RoC commandwhich is issued and used for the determination of the pitch command andtilt command, according to an embodiment of the invention.

The rate of climb management module 1221.2 issues the RoC command (whichmay indicate the value of required rate-of-climb) during theacceleration phase. Usually the command is set to zero below apredefined airspeed limit (in order to initially use energy toaccelerate the air vehicle, e.g., lower limit of 15-20 knots). The RoCcommand may be set linearly above such limit (e.g., 50 ft/min), butother, non-linear, RoC control schemes may also be implemented. In caseswhere the air vehicle crosses the desired flight path by more than apredefined threshold (e.g. 5 meters) towards the ground, dZ loop 1221.11output may optionally be summed to the nominal RoC command to ensureproper ground clearance.

Optionally, the rate-of-climb command may be automatically updated toavoid conflicts (if any) with the desired flight path angle (γ). Forexample, if the desired flight path is positive (i.e., ascending), thenthe desired rate-of-climb command in such an implementation must be alsopositive, and in correlative value. On the other hand, since dZ loop isonly engaged if the air vehicle is below the desired altitude, it ispossible to set positive desired rate-of-climb and negative flight pathangle (γ), but not vice-versa.

Rate-of-climb loop 1261.3 may be implemented for controlling the airvehicle heave (vertical) speed. Inputs to the module are rate-of-climbcommand, measured rate-of-climb, and rate-of-climb derivative (which isheave acceleration, Az). Module output is a preliminary throttlecommand, which may then be summed with the nominal throttle value fortakeoff (a function of the airspeed itself, as explained above). Theimplementation of the rate-of-climb controller illustrated in FIG. 15 isclassical PID (Proportional-Integral-Differential) control law.

FIG. 17 illustrates interrelations between various components of system1201, and especially the inner control loop 1260, according to anembodiment of the invention.

Tiltable propulsion unit plane management module 1262 may be configuredto coordinate the air vehicle angle of attack with changes in airspeed,aircraft pitch, and so on. The angle of attack should be maintained atall times in a tight “corridor”: on one hand, it should allow the airvehicle to accelerate, hence minimal drag is necessary, facilitating alower angle of attack. On the other hand, it is highly desirable toreceive positive lift from the wings in order to alleviate the motorsload and energy consumption. Furthermore, the angle of attack should notexceed stall (positive or negative) at any time.

Pitch control module 1224.1 may be configured to control in the firstpart of the acceleration a modifying of a pitch angle of the airvehicle, and to control in the first part of the acceleration amodifying of a pitch angle of the air vehicle, thereby preventingcreation of negative lift force by the wing.

Pitch control module 1224.1 may be configured to control in the firstpart of the acceleration a modifying of a pitch angle of the air vehicleand to limit in the first part of the acceleration lift created by thewing below a given fraction (e.g., 80%) of maximal lift which may becreated by the wing in the monitored airspeed. That is, at any givenspeed V, the system does not utilize the maximum lift possible by thewing, but rather provides more lift by the tiltable propulsion unit.

Tiltable propulsion unit plane management module 1262 may be configuredto achieve this balance by summing three inputs:

-   -   Tilt takeoff bias (e.g.) −10°, which engages upon switching to        “accelerate” mode (motors forward 10°).    -   Pitch command from pitch control module, multiplied by a        constant gain (lower pitch=motors more forward), e.g., of about        1 degree forward per degree pitch down command    -   Measured airspeed, multiplied by a constant gain (e.g. of about        0.33, i.e., 10° over 30 knots)

The sum of the above inputs goes through a rate limiter 1262.1 (e.g.,about 1-2°/sec) and drives the tilt mechanism servo actuators. Thepurpose of this rate limiter 1262.1 is to suppress gyroscopic effectsand gusts (unstable measured airspeed) and to avoid fast changes in theair vehicle dynamics, enabling the control loops to gain betterperformance.

In the illustrated example, control unit 1290 (and especially the innercontrol loop) controls the tilt by controlling all three motors of anair vehicle 100 such as the ones illustrated in FIGS. 1A, 1B, or inFIGS. 2A and 2B.

As aforementioned, the tilt command is determined based on the monitoredairspeed and on the airspeed command and optionally further based on thepitch command Optionally, pitch control module 1224.1 may be configuredto issue a pitch command based on the monitored airspeed and theairspeed command and to control in the first part of the acceleration amodifying of a pitch angle of the air vehicle based on the pitchcommand, wherein the controlling of the modifying of the tilt angle isfurther based on the pitch command. Optionally, control unit 1290 (e.g.by tiltable propulsion unit plane management module 1262) may beconfigured to issue a rotor tilt command for lowering a rotor tilt angleas a result of an issue of a pitch command for lowering a pitch angle.

FIG. 18 illustrates interrelations between various components of system1201, and especially the inner control loop 1260, according to anembodiment of the invention. Optionally, roll management module 1224.2is configured to issue the roll command based on the sideways velocitycommand, and on a roll limiter 1224.21 which may implement one or moreairspeed based limiters—a lower limit, an upper limit, or both.

Referring to system 1200, and to the various illustrations in FIGS. 9,14, 15, 17, 18, it is noted that, while not necessarily so, the gains ofthe control-loops may be determined based on stage in process (i.e., onthe command of process management module 1240), and not on parameters ofthe platform. That is, the gains (or other control schemes) aredetermined not as a result of changes in the behavior of the airvehicle, but in order to change the behavior of the air vehicle!

Control unit 1290 may be configured so as to control the acceleration ofthe air vehicle, while minimizing time in which the one or more tiltablepropulsion units provide thrust in the general vertical thrust vectordirection. Control unit 1290 may include a tilting control module thatis configured to determine a timing for tilting of the at least onetiltable propulsion unit between the second and the first parts of theacceleration for minimizing a duration in which the tiltable propulsionunit provides thrust in the general vertical thrust vector direction.

As discussed in greater detail with respect to stage 3100 of method3000, control unit 1290 may be configured to control, before the firstpart of the acceleration, a substantially vertical ascent of the airvehicle from the ground, and to control aerodynamic parts of the airvehicle in a first part of the substantially vertical ascent from theground for minimizing change in pitch angle and roll angle of the airvehicle.

Control unit 1290 may also be configured to issue a positive airspeedcommand for accelerating the air vehicle if predetermined time elapsedfrom an initiation of the substantially vertical ascent, regardless ofsensors data. Other such responses which are based on time rather thanon sensors data may also be implemented for other cases, e.g., asdiscussed with respect to method 3000.

Pitch control module 1224.1 may be configured to control in the firstpart of the acceleration a modifying of a pitch angle of the airvehicle, wherein the control unit is configured to balance betweenmodifying the tilt angle and modifying the pitch angle for reducing anenergy cost of the acceleration of the air vehicle.

Referring to FIG. 1A, it is noted that control system 1201 (discussedabove in greater detail, e.g. with respect to FIG. 9) may be integratedinto air vehicle 100. Such integration may include, for example, any oneor more of the following: fixing components of system 1201 to the airvehicle 100, electrically connecting components of the system 1201 toair vehicle 100, connecting components of system 1201 to a communicationmedium (e.g., a communication bus) of the air vehicle 100, sharingcomponents between system 1201 and other systems of air vehicle 100(e.g. processors used for the functionalities of system 1201 may beprocessors of air vehicle 100 which are used for other functionalitiesthereof), providing electrical power to units of system 1201 from apower source of air vehicle 100, and so on.

That is, according to an aspect of the invention, an air vehicle 100 isdisclosed, which includes at least: (1) wing 320, (2) tiltablepropulsion unit tiltable propulsion unit 420 (which is tiltable toprovide a thrust whose direction is variable at least between a generalvertical thrust vector direction and a general longitudinal thrustvector direction with respect to the air vehicle); and (3) control unit1201 which is configured to issue controlling commands to a controller1410 of the tiltable propulsion unit tiltable propulsion unit 420 forcontrolling the acceleration of the air vehicle.

This controlling of the acceleration includes: (1) in a first part ofthe acceleration, in which the tiltable propulsion unit provides thrustin the general vertical thrust vector direction: (a) controlling anoperation of the at least one tiltable propulsion unit for providinglift to the air vehicle, and (b) controlling a modifying of a tilt angleof the tiltable propulsion unit with respect to a fuselage of the airvehicle, based on: (i) a monitored airspeed of the air vehicle and (ii)an airspeed command; and (2) following a tilting of the at least onetiltable propulsion unit, controlling in a second part of theacceleration an operation of the tiltable propulsion unit to providethrust in the general longitudinal thrust vector direction forpropelling the air vehicle. Variations discussed above with respect toair vehicle 100, as well as variations discussed above with respect tocontrol system 1201, may be implemented in such a combinedimplementation.

FIG. 10 is a flow chart of method 3000, which is a method forcontrolling an acceleration of an air vehicle (such as air vehicle 100)which includes a tiltable propulsion unit that is tiltable to provide athrust whose direction is variable at least between a general verticalthrust vector direction and a general longitudinal thrust vectordirection with respect to the air vehicle, according to an embodiment ofthe invention.

During the discussion of method 3000, only one out of the one or moretiltable propulsion units which may be implemented in the air vehicle isexplicitly referred to. In implementations in which the air vehicleincludes a plurality of tiltable propulsion units, any one or more ofthese units may be controlled according to method 3000. The discussionexplicitly refers only to one tiltable propulsion unit for reasons ofclarity and simplicity of the description, and it is noted that theinvention is not limited to such an implementation in any way.

It should be noted that the velocity profile of the air vehicle duringthe acceleration of method 3000 is not necessarily a strictly monotonicaccelerating one, and that while a speed of the air vehicle at the endof the acceleration is substantially higher than its speed at thebeginning of the acceleration process, the air vehicle may neverthelessexperience some temporary decelerations (e.g. due to unexpected winds orair conditions, or due to moving of control surfaces of the air vehicle,and even as effects of actions taken as part of the controlling executedin method 3000—e.g. in order to keep the air vehicle within an envelopethat ultimately permits acceleration to a speed in which the tiltablepropulsion unit provides thrust in the general longitudinal thrustvector direction). The accelerating may include acceleration of the airvehicle in a substantial fraction thereof—e.g. for 80% of its duration,and possibly even more (e.g. 90%, 95%, etc).

While not necessarily so, the acceleration of method 3000 may be part ofa take-off process (or an equivalent climbing and accelerating from aprevious hover state). In such cases, the acceleration of method 3000may be concurrent (fully or partly) to a climbing of the air vehicle.However, in other situations the acceleration of method 3000 may beconcurrent to another height change profile (e.g., height decreasing oralternating ascend/descend). While descending from a landing position isimpossible in most situations, accelerating from a stand-still whiledescending has advantages if possible—e.g., for transforming potentialheight energy to kinetic energy, thereby facilitating a quicker switchto flight in the more efficient first flight mode).

Referring to the examples set forth in the previous drawings, it isnoted that method 3000 may be implemented to control an accelerationprocess of an air vehicle such as air vehicle 100. However, this is notnecessarily so, and method 3000 may also be used to control decelerationprocesses of other types of air vehicles including one or more tiltablepropulsion units 420.

Referring to the examples set forth in the drawings, it is noted thatwhile not necessarily so, method 3000 may be carried out by a controlsystem such as control system 1201. In various implementations, method3000 may be carried out—wholly or partly, by an on-board system, by aremote system (e.g. ground system or airborne system), and/or by humanintervention, as well as by any combination thereof.

It should be noted that in various implementations of method 3000, theaccelerating process may address different needs, and may enableexecution of method 3000 (and/or of specific stages thereof) withinconstraints that are stricter than previously possible. For example,while prior art tiltrotor aircrafts are known to have accelerated andeven take-off, doing so in a sufficiently quick manner, within asufficiently small distance, and/or gaining substantial height and/orvelocity in the process, this may turn out to be impractical using priorart schemes. Some examples of scenarios are provided below.

It is noted that method 3000 includes several stages of controlling, asis disclosed below in detail. Such controlling may be implemented invarious ways. Such controlling may be implemented by a pilot, by anotherperson onboard, and by a remote human operator (e.g. for an unmannedtilt rotor air vehicle). However, method 3000 may also be implemented byone or more computerized systems (e.g. as exemplified in relation tosystem 1201). Such a system may be mounted onboard the air vehicle ofmethod 3000, or externally, and multiple such systems may coordinate toimplement method 3000 (wherein each stage of the method may beimplemented by a single system or a combination of such computerizedsystems). Additionally, a combination of one or more human controllersand one or more computerized systems may also be implemented.

According to an embodiment of the invention, the controlling of the airvehicle during method 3000 includes automated controlling by at leastone processor of a control unit mounted on the air vehicle. It is notedthat such processors and/or other computerized systems may be adedicated system (implemented in hardware, firmware, etc.), and may alsobe implemented in software run by a processor of another system mountedon the air vehicle.

It is also noted that different stages of method 3000 includecontrolling operations. While not necessarily so, in each of thecontrolling stages method 3000 may possibly also include the carryingout of the controlled operation, even if not explicitly elaborated so.By way of example, on top of the controlling of the tilt angle of thetiltable propulsion unit with respect to a fuselage of the air vehiclein stage 3220, method 3000 may further include modifying the tilt angleof the tiltable propulsion unit with respect to a fuselage of the airvehicle.

Controlling of the course/position/kinetic parameters/location/directionof the air vehicle during method 3000 may be achieved at least bycontrolling an operation of one or more of the aerodynamic subsystems ofthe air vehicle. Such parts may include, by way of example, the at leastone tiltable propulsion unit, at least one non-tiltable propulsion unit,a throttle, an engine, ailerons, elevators, rudder, ruddervator,flaperons, elevons, wing flaps, slats, spoilers, air brakes,variable-sweep wings, non-tiltable propulsion unit, blades of rotors,and so on. It is noted that different stages of method 3000 may includecontrolling an operation of at least one aerodynamic subsystem of theair vehicle selected from a group consisting of an aileron, an elevator,a rudder, a ruddervator, a flaperon, elevons, and a wing flap.

The controlling of such aerodynamic subsystems (and/or other parts) maybe achieved in various ways, such as by issuing instructions to suchparts, or to components controlling such parts. In a few exemplaryimplementations, instructions may be implemented by modifying anelectric current transmitted to servos controlling such parts, byinstructing a hydraulic pump to modify a pressure in a pipe leading tosuch a part, and so forth. In other examples, the controlling may beachieved by physical means. For example, if method 3000 is wholly orpartly carried out by a pilot (or other person onboard), the pilot maychange a physical state of one or more component—e.g. push a throttle.It is noted that physical means for controlling the course may also beimplemented by systems and not only by humans, as will be clear to aperson who is of skill in the art.

It should be noted that method 3000 may include not only the controllingof the operation of one or more of the aerodynamic subsystems of the airvehicle or other components/systems thereof, but also actual operationthereof. In an example, while the controlling may be carried out by oneor more people, processors, controllers, or like systems (differentimplementations and combinations thereof are possible), the operating ofthe different parts/components/systems of the air vehicle may be carriedout by other parts/components/systems mounted on the air vehicle.

Referring to the course of the air vehicle during the execution ofmethod 3000, it should be noted that the controlling of the course ofthe air vehicle in method 3000 may include controlling temporal and/orspatial aspects of it. For example, the controlling may includecontrolling of some or all of the following parameters—the speed of theair vehicle (or components thereof such as groundspeed, airspeed,descending speed, and so forth), controlling its arrival at apredetermined location at a certain time, controlling its altitude, itshorizontal positioning, its pitch, its turn, its yaw, its direction, andso on and so forth.

The controlling of the course may include controlling the course atleast for keeping the air vehicle within an envelope that ultimatelypermits its acceleration to a speed which permits flight in the firstflight mode, or which ultimately permits accomplishing another goal. Itis noted that such an envelope may not be the largest envelopepermitting such a condition, but rather an envelope defined in view ofsuch a goal. Some or all of the parameters defining such an envelope mayalso be defined regardless of the final destination, e.g. resulting fromaerodynamic considerations (for example prevention of reaching astalling angle, keeping direction against the wind), from tacticalrequirements (e.g. reducing an exposure period above/below givenheight), for requirements of another system of the air vehicle or systemcarried by it (e.g. for preventing damage to a sensitive camerapayload), and so forth.

While the term envelope is a term widely used in the art and, asaforementioned, may carry meaning as understood by one of ordinary skillin the art, it is noted that this term may be regarded as including atleast one or more of the following sets of parameters: a set ofperformance limits (e.g. of the aircraft) that may not be safelyexceeded, a set of operating parameters that exists within these limits,and a set of spatial and/or temporal parameters relating to courseparameters.

It is noted FIG. 20 illustrates a possible exemplary flight course ofair vehicle 100, during which it accelerates, according to an embodimentof the invention. The parts of that flight course during which severalstages and sub-stages of method 3000 may be executed (by way of anon-limiting example only) are indicated in FIG. 20.

Reverting to FIG. 10, method 3000 is executed over at least twosignificant parts of the acceleration. In the first part of theacceleration, the tiltable propulsion unit of the air vehicle providesthrust in the general vertical thrust vector direction, and in thesecond part of the acceleration, the tiltable propulsion unit providesthrust in the general longitudinal thrust vector direction.

Stage 3200 is carried out in the first part of the acceleration, inwhich the tiltable propulsion unit provides thrust in the generalvertical thrust vector direction. Stage 3200 includes at least stage3210 and stage 3220.

Stage 3210 includes controlling an operation of the at least onetiltable propulsion unit for providing lift to the air vehicle.Referring to the examples set forth with respect to the previousdrawings, stage 3210 may be carried out by a tiltable propulsion unitcontroller such as tiltable propulsion unit controller 1410. Asaforementioned, the providing of the lift is carried out when the one ormore tiltable propulsion units are directed at the general verticalthrust vector direction. Since the one or more tiltable propulsion unitsmay provide thrust in a non-vertical direction (due to tilting of suchunit and/or to an angle in which the air vehicle itself is position), itis noted that concurrent to the providing of the lift in stage 3210, thevery same one or more tiltable propulsion units may also propel the airvehicle (e.g. equivalent to the manner in which the rotor of ahelicopter might propel the helicopter, even though it provides thrustin the general vertical thrust vector direction).

The controlling of the at least one tiltable propulsion unit forproviding lift may be executed (e.g., as discussed below) according to acontrol scheme dictating a flight course. For example, in an ascendingcourse the overall lift of the air vehicle has to be a positive lift.However, some of the lift—especially when the air vehicle is in aforward motion, comes from the wings and from other parts of the airvehicle, and not only from the tiltable propulsion unit.

Stage 3220 includes controlling a modifying of a tilt angle of thetiltable propulsion unit (or of more than one such units, ifimplemented) with respect to a fuselage of the air vehicle, based on:(i) the monitored airspeed and (ii) an airspeed command Referring to theexamples set forth with respect to the previous drawings, stage 3220 maybe carried out by a tiltable propulsion unit controller such as tiltablepropulsion unit controller 1410. It is noted that the controlling of themodifying of the tilt angle is a continuous process in which the tiltangle is changed many times, due to changing conditions (e.g.acceleration of the air vehicle, changing winds, changes in plannedflight course, etc.).

It is noted that the controlling of stage 3220 is based on the monitoredairspeed as such, and not exclusively (if at all) on derivativeparameters such as airspeed error (which is the difference between amomentary monitored airspeed and a planned airspeed for that moment).Especially, optionally, the controlling of stage 3220 may be executedbased on the monitored airspeed independent of the airspeed error.

The controlling of stage 3220 is also based on the airspeed command. Theairspeed command itself may be based on a preplanned accelerationfunction. For example, the airspeed command may instruct for a moment Tairspeed V so that V=f(T) (i.e. irrespective of other factors). Forexample, the airspeed command may instruct airspeed V=V₀+a×T.

As an optional way of being based on the airspeed command, thecontrolling of stage 3220 may be based on a parameter which is in turnbased on the airspeed command, such as an airspeed error (an error ofthe monitored airspeed with respect to the airspeed command). That is,optionally the controlling of the modifying of the tilt angle in stage3220 may be based on (i) the monitored airspeed; and (ii) the airspeederror.

Some of the ways in which the controlling of stage 3220 may depend onthese different factors are discussed below in greater detail.

However, method 3000 may include controlling, monitoring and/or managingadditional parameters of the air vehicle during the first part of theacceleration (i.e., concurrently—fully or partly—to the execution ofstages 3210 and 3220).

Stage 3200 may include any of the following stages (and others):

-   -   Stage 3230 of controlling pitch of the air vehicle, based        on: (i) the monitored airspeed and (ii) an airspeed command        Referring to the examples set forth with respect to the previous        drawings, stage 3230 may be carried out by a pitch control        module such as pitch control module 1224.1.    -   Stage 3240 of controlling a throttle of the at least one        tiltable propulsion unit (or otherwise controlling its output        thrust), based on the airspeed command and/or on a rate-of-climb        (RoC) command. Referring to the examples set forth with respect        to the previous drawings, stage 3240 may be carried out by a        throttle management module such as throttle management module        1261.

Apart from controlling the various components of the air vehicle asmentioned above, method 3000 may also include issuing control commandsduring the first part of the acceleration. That is, stage 3200 mayinclude any of the following stages (and others):

-   -   Stage 3250 of issuing the airspeed command. Referring to the        examples set forth with respect to the previous drawings, stage        3250 may be carried out by a longitude airspeed management        module such as longitude airspeed management module 1222.1.    -   Stage 3260 of issuing the RoC command. Referring to the examples        set forth with respect to the previous drawings, stage 3260 may        be carried out by a climb rate management module such as climb        rate management module 1221.2. It is noted that stage 3260 may        include issuing an altitude command instead of (or in addition        to) the RoC command, and the modules which are described as        utilizing the RoC command may utilize the altitude command.    -   Stage 3270 of issuing a pitch command. Referring to the examples        set forth with respect to the previous drawings, stage 3270 may        be carried out by a pitch control module such as pitch control        module 1224.1.

It is noted that stages 3250, 3260, and/or stage 3270 may be executedcontinuously, issuing different commands at different times during thefirst part of the acceleration.

Referring to stage 3270, it is noted that method 3000 may includeissuing the pitch command based on the monitored airspeed and on theairspeed command. Optionally, stage 3200 may further include stage 3280of controlling, in the first part of the acceleration, a modifying of apitch angle of the air vehicle, based on the pitch command Referring tothe examples set forth with respect to the previous drawings, stage 3280may be carried out by a pitch control module 1224.1 such as pitchcontrol module 1224.1. Optionally, the controlling of the modifying ofthe tilt angle in stage 3200 is further based on the pitch command.

There are many ways in which various commands (e.g., pitch, altitude,tilt, RoC, etc.) may be issued (i.e., selected, determined, etc.) andlater utilized for controlling various components of the air vehicle.

It is noted that the issuing of the various control commands may be aninterrelated process, in which one command is issued based on anothercommand, thereby taking into account its anticipated effects.

For example, the controlling of the modifying of the tilt angle in stage3220 may include issuing a rotor tilt command (according to which themodifying of the tilt in stage 3220 is executed and controlled) furtherbased on the pitch command. Stage 3220 may include issuing a rotor tiltcommand for lowering a rotor tilt angle as a result of an issue of apitch command for lowering a pitch angle. Stage 3220 may include issuinga rotor tilt command for raising a rotor tilt angle as a result of anissue of a pitch command for raising a pitch angle.

In such cases, the overall angle in which a plane of the tiltablepropulsion unit (e.g., its rotor plane, if applicable) is tilted (raisedor lowered) with respect to the horizon—the overall angle effecting thedirection of the thrust and therefore the lift and the propulsiongenerated by the respective tiltable propulsion unit—is combined fromtwo complementary changes of directions: the pitch of the entire airvehicle, and the angle of the aforementioned plane of the tiltablepropulsion unit with respect to the air vehicle.

It is noted that this is not necessarily so, and in other cases suchtilting of the plane of the tiltable propulsion unit may be achieved bytilting either the pitch plane of the fuselage or the tilt plane of thetiltable propulsion unit with respect to fuselage (but not both, in suchalternatives), and even by changing the pitch angle and the tilt anglein the opposing direction.

Optionally, the issuing of the rotor tilt angle during the first part ofthe accelerating may be limited to issuing a rotor tilt command forchanging the rotor tilt angle in a given direction (raising or lowering)as a result of an issue of a pitch command for changing of the pitchangle in the same direction.

It is noted that different relations between the pitch and tilt commandaffect both the angle of the wings (and hence the lift generated bythem) and the angle of the tiltable propulsion unit (and hence the liftand/or propulsion generated by it). Therefore, different control schemesmay be implemented for achieving different goals. For example, stage3200 may include controlling in the first part of the acceleration amodifying of a pitch angle of the air vehicle, and balancing betweenmodifying the tilt angle and modifying the pitch angle for reducing anenergy cost of the acceleration of the air vehicle.

It is noted that additional goals may also be reached by the samecontrol scheme. For example, while the control scheme may be used forefficient acceleration of the air vehicle, it may also attempt to keep a“best” angle of attack of the wing, while reducing overall energy costs.

It should be noted that when considering the state of the air vehiclecontrols—controlling speed and direction of tiltable propulsion unit,direction of the wing, flaps, elevators, etc.—in order to prevent theair vehicle from pitching (or for controllably changing its pitch),there are many solutions which give Σmoments=0 (thus not pitching theaircraft, or another desired Σmoments). However, only a few of themwould enable proper acceleration and keeping of desired lift. Some suchsolutions may be reached by following the above rules.

During the acceleration, a gradual transfer of the support of the weightof the air vehicle and of the positive lift from the motors of the airvehicle (and especially from the motors of the at least one tiltablepropulsion unit) to the wing is executed and controlled. Such gradualtransfer is executed so that during an ascending acceleration course ofthe air vehicle the sum of the lift generated by the thrust of thetiltable propulsion unit and by the wing is greater than the weight ofthe air vehicle, and so that it leads to a state in which the wing (andpossibly additional surfaces of the air vehicle) may generate lift whichis equal to or larger than the weight of the air vehicle (during thetransition). During the transition of the one or more tiltablepropulsion units the lift does not fall below the weight of the airvehicle.

Referring to the issuing of the pitch command (on which the controllingof stage 3230 is based, either exclusively or not), it is noted thatpitch command may be issued based on additional parameters, in additionto the aforementioned airspeed command and monitored airspeed. Forexample, the issuing of the pitch command may further be based on themeasured altitude of the air vehicle (directly as a function of themeasure altitude, i.e., CMD_(Pitch)=F(ALT_(measured)) and/or on aderivate parameter such as altitude error, which is a difference betweenthe measured altitude and the altitude indicated in the altitudecommand.

As aforementioned, there are many ways in which various commands (e.g.,pitch, altitude, tilt, RoC, etc.) may be issued and later utilized forcontrolling various components of the air vehicle. For example, aprocess of issuing and utilizing commands which may be part of method3000 (and especially of stage 3200) may include the following steps(e.g., in the following order):

-   -   Issuing the airspeed command (stage 3250);    -   Issuing the pitch command based on the airspeed command (and        possibly also on additional data such as the monitored airspeed,        stage 3270);    -   Controlling effectors of the air vehicle (e.g. motors, ailerons,        elevators, rudder, wing flaps, slats, etc.), and especially the        at least one tiltable propulsion unit based on the airspeed        command and the pitch command (and possibly on additional data,        such as though not necessarily limited to the monitored        airspeed). This may include any of stages 3210, 3220, 3230,        3240, and 3280, as well as controlling of other components of        the air vehicle.    -   It is noted that stages 3210, 3220, 3230, 3240, and 3280 may        have synergetic/contradicting effects when executed        concurrently, and therefore any two or more of these stages may        be executed based on a higher level control (e.g., by one or        more of the navigational parameters management modules 1250);

Another process of issuing and utilizing commands which may be part ofmethod 3000 (and especially of stage 3200) and which may be executedconcurrently (fully or partly) with the previous process may include thefollowing steps (e.g., in the following order):

-   -   Issuing an altitude/RoC command (stage 3260);    -   Possibly issuing a correction command (which may be        added/replace the altitude/RoC command), based on flight        parameters of the air vehicle (e.g. height above ground, climb        speed, etc.) and on allowed envelope;    -   Issuing a throttle command based on the altitude/RoC command and        possibly on additional data (e.g., momentary tilt angle and        pitch);    -   Controlling the throttle of the at least one tiltable propulsion        unit based on the throttle command (stage 3240).

Additional discussion in which the ways such commands may be issued andutilized as part of method 3000 is offered with respect to the variouscontrol modules of system 1201.

The issuing and/or utilization of any of the above control commands forthe controlling of the course of the air vehicle during the acceleration(and likewise in other stages of its flight) may be subject to variouskinds of limiters. Such limiters may be implemented in order to preventthe air vehicle from entering various dangerous states (e.g., crashinginto the ground, overreacting to winds, stalling, etc.). Such limitersmay be of various kinds. For example, the limiters may be time limiters,quantity limiters, rate limiters, and so on.

Optionally, method 3000 (e.g., as part of stage 3200, and especially aspart of stage 3240) may include controlling thrust power of the tiltablepropulsion unit for reducing a difference between the measured airspeedand a set airspeed, while restricting increasing of the thrust powerbased on a threshold (4110) that is determined in response to themeasured airspeed. That is, even if the condition of reducing (or evendiminishing, if possible) the difference between the measured airspeedand the airspeed command at a given moment may be reached by increasingthe thrust power of the tiltable propulsion unit by a given amount, suchincrease would be avoided if it would violate the threshold (whichitself is based on the measured airspeed).

FIG. 11 is a graph which illustratively exemplifies restriction ofthrust power based on the measured airspeed of the air vehicle duringthe acceleration, according to an embodiment of the invention. Forexample, the aforementioned restricted controlling of the thrust powerduring the acceleration may include restricting increasing of the thrustpower based on a higher threshold 4110 that is determined in response toa measured airspeed of the air vehicle. Optionally, a lower thresholdmay also be implemented.

Referring to FIG. 11, higher threshold 1710 represents the maximalpermitted thrust in any possible measured airspeed, and lower threshold4120 represents the minimal permitted threshold in any possible measuredairspeed. Line 4130 represents a possible thrust during an exemplaryacceleration of the air vehicle. It is noted that different thrusts areshown in line 4130 for a single airspeed, as may occur in differentsituations—e.g. due to winds or due to the fact that the deceleration isnot necessarily monotonic, and therefore the air vehicle may measure thesame airspeed more than once. It is noted that the above discussionpertaining to thresholds 1710 and 1720 is also applicable for thresholds4110 and 4120, mutatis mutandis. For example, the thresholds (both lowerand higher) may not necessarily be a thrust threshold, but rather may bethresholds of one or more parameters associated with thrust (especiallywith thrust in the general vertical thrust vector direction), such as byway of example a threshold of the power provided (or sourced) by the atleast one tiltable propulsion unit for providing thrust in the generalvertical thrust vector direction. A controlled value (e.g. throttlestate) or a measured value (e.g. measured rotation speed) may be easierto control and keep within an allowed limit than controlling the thrustdirectly. It is noted that controlling of the thrust power may beimplemented by controlling such a controlled parameter (such as thepower provided to the tiltable propulsion unit or the state of thethrottle).

While not illustrated, it is noted that the rate of increasing thethrust may also be limited. For example, even if the low threshold isexceeded for whatever reason, the thrust according to such animplementation may not be sharply increased, but rather increased in acontrolled manner wherein the increase rate does not exceed apredetermined value. The decreasing rate of the thrust may or may not besimilarly limited.

It is noted that other parameters may also be monitored and restrictedbased on applicable thresholds. Among such parameters are: altitude,RoC, pitch, rate of pitch change, yaw, rate of yaw change, roll, rate ofroll change, direction with respect to the wind and its rate of change,thrust provided by any non-tiltable propulsion unit, etc.

For example, optionally, method 3000 may include keeping a pitch of theair vehicle during at least a part of the first part of the accelerationwithin a permitted pitch range that is dynamically determined inresponse to the measured airspeed of the air vehicle. This isillustrated in FIG. 12, in which a lower threshold 4220 and a higherthreshold 4210 define that permitted pitch range in different airspeeds.Line 4130 represents possible pitch during an exemplary acceleration ofthe air vehicle. It is noted that the keeping of the pitch within thepermitted pitch range may be implemented for preventing stalling of theair vehicle.

The controlling of the flight of the air vehicle, and especially of itsacceleration, may be implemented using many decision rules for thedetermining and/or modifying of many parameters—some of which directlyaffect the state of various control modules of the air vehicle, andothers indirectly. In some situations, different such decision rules maylead to contradicting results. For example, an air speed module mayinstruct to lower the tilt of the tiltable propulsion unit and increaseits thrust power while a pitch module may instruct at the same time toincrease the tile of the same tiltable propulsion unit. Therefore,different resolve mechanisms may be implemented to decide how suchconflicts are resolved. One solution is that one rules overrides anotherrule (at least in some defined circumstances, e.g., when the pitch isbetween α₁ and β₁ degrees).

Referring to the thrust threshold discussed with reference to FIG. 11,possibly the thrust thresholds over-rides airspeed command Stage 3200may include controlling thrust power of the tiltable propulsion unit forreducing a difference between the measured airspeed and a set airspeed,while restricting reduction of the thrust power based on a lowerthreshold (threshold 4120 in the example of FIG. 11) that is determinedin response to a measured airspeed of the air vehicle.

Referring to the thrust threshold discussed with reference to FIG. 11,possibly the thrust thresholds over-rides any altitude or RoC commands(at least in some altitude range). Stage 3200 may include minimizing avertical deviation of the air vehicle from a set altitude during thefirst part of the acceleration, wherein the minimizing is restricted atleast by the restricting of the reduction of the thrust power based onthe lower threshold.

Referring to the control of the air vehicle during any stage of itsflight, and especially to the control of the air vehicle during theacceleration, the overall lift applied to the air vehicle during itsflight results both from lift generated by the tiltable propulsion unitsand by lift which is generated by the wing and additional surfaces (e.g.fuselage), as long as the air vehicle is flying at non-zero airspeed.

When the air vehicle has zero airspeed, practically the entire lift isgenerated by the tiltable propulsion unit. However, when the air vehicleaccelerates, more lift may be generated by the wing. At any givenairspeed, the amount of lift generated by the wing depends greatly onthe pitch of the air vehicle, and therefore the ratio between the amountof lift generated by the wing to the amount of lift generated by thetiltable propulsion unit also depends on the pitch (and on additionalparameters, such as tilt of the at least one tiltable propulsion unitand the state of its throttle).

A given combination of rate of climb and airspeed (and possibly ofadditional parameters as well) may therefore be reached and/ormaintained using a different ratio between lift generated by the wind tolift generated by the tiltable propulsion unit. Pitching the air vehicleso that its wing would provide the maximal possible lift at a givenairspeed would require minimal relative lift to be generated by the oneor more tiltable propulsion units, and therefore, more power of thetiltable propulsion unit may be channeled to acceleration.

Stage 3230 is nevertheless optionally implemented so that it includescontrolling in the first part of the acceleration a modifying of a pitchangle of the air vehicle, thereby limiting lift created by the wingbelow 80% of maximal lift which may be created by the wing in themonitored airspeed. That is, at any given airspeed, the controllingincludes not utilizing the maximum lift possible by the wing, but ratherproviding more lift by the one or more tiltable propulsion units.

Stage 3230 may also include controlling in the first part of theacceleration a modifying of a pitch angle of the air vehicle, therebypreventing creation of negative lift force by the wing. Stage 3230 mayalso include controlling in the first part of the acceleration amodifying of a pitch angle of the air vehicle, thereby reducing drag.

Method 3000 may include controlling a tilting of the at least onetiltable propulsion unit (e.g. to a position in which it provides thrustvector in the general longitudinal thrust direction). For example, thecontrolling of stage 1530 may include controlling the tilting of the atleast one tiltable propulsion unit to a position in which the at leastone tiltable propulsion unit provides thrust vector whose component(projection) in the general longitudinal thrust direction issubstantially larger than its component (projection) in the generalvertical thrust vector direction (e.g. at least 2 times larger, at least5 times larger, at least 20 times larger, and so on). Referring to theexamples set forth in the previous drawings, stage 3300 may be carriedout by a control unit such as tiltable propulsion unit controller 1410.It is noted that method 3300 may include a stage of tilting the at leastone tiltable propulsion unit, e.g. to a position in which it providesthrust vector in the general longitudinal thrust direction.

As aforementioned with respect to the previous drawings (e.g. withrespect to method 1500), the tilting may be carried out in differentways, and according to different schemes. It is noted that the tiltingof method 3300 may be controlled and executed in any of the alternativesdiscussed with respect to method 1500 (but in the other direction). Itis noted that various such ways and schemes may be performed in a singlesystem implementing method 3300, wherein in each case the actual schemeto be implemented may be selected, for example, according toenvironmental conditions, aerodynamic conditions, a state of the airvehicle, and so forth.

For example, the controlling of the tilting may include controlling atiming of the tilting (e.g. in respect to the fastest possible tiltingtime in a given system), a degree of the tilting, the number and orderof tiltable propulsion units tilted—and to what degree, tiltingdifferent units concurrently, partly concurrently, or sequentially, anoperation of a tiltable propulsion unit during its tilting (e.g. thrustprovided by it), and so forth.

Method 3300 may further include an optional stage of determining timingfor tilting of the at least one tiltable propulsion unit between thefirst and the second parts of the acceleration for minimizing a durationbetween the initiation of the acceleration and the tilting. Method 3300may further include an optional stage of determining timing for tiltingof the at least one tiltable propulsion unit between the second and thefirst parts of the acceleration for minimizing a duration in which thetiltable propulsion unit provides thrust in the general vertical thrustvector direction. Clearly, if such a stage is carried out, thecontrolling of the tilting in stage 3300 may be responsive to thedetermined timing. Method 3300 may include a stage of providing thetiming determined in such a stage to at least one unit that participatesin the controlling of the tiling of the at least one tiltable propulsionunit.

The timing of the tilting depends at least in part on the airspeed ofthe air vehicle. Once tilted, most of the thrust generated by the one ormore tilted tiltable propulsion units, propels the air vehicle ratherthan generates lift. Therefore, the tilting must be made when theairspeed is sufficient for providing most (if not all) of the lift bythe wing—similar to a regular fixed-winged aircraft. In order to avoidstalling, the tilting may be timed to a time when the airspeed exceedsto a sufficient extent the stall speed of the air vehicle.

Stage 3400 of method 3000 includes controlling in a second part of theacceleration, following a tilting of the at least one tiltablepropulsion unit, an operation of the tiltable propulsion unit to providethrust in the general longitudinal thrust vector direction forpropelling the air vehicle. Referring to the examples set forth withrespect to the previous drawings, stage 3400 may be carried out by atiltable propulsion unit controller such as tiltable propulsion unitcontroller 1410.

Once all of the one or more tiltable propulsion units of the air vehicleare tilted, the air vehicle may continue in a flight in the first flightmode, and accelerate or decelerate, based on need.

Reverting to stage 3200, it is noted that before the acceleration, theair vehicle may either be stationary (e.g. landed or hovering), orflying in the second flight mode. It is noted that while stage 3200 maybe initiated when the air vehicle is landed on the ground, a preliminarytake-off stage may be implemented, in which a different control schemethan the one of stage 3200 is used for controlling the air vehicle.

Method 3000 may include stage 3100 of controlling, before the first partof the acceleration, a substantially vertical ascent of the air vehiclefrom the ground. Stage 3100 may include stage 3110 in which in a firstpart of the substantially vertical ascent from the ground, controllingaerodynamic parts of the air vehicle for minimizing change in pitchangle and roll angle of the air vehicle. At the first part of thesubstantially vertical ascent, the air vehicle is relatively close toground, and therefore may hit the ground if trying to maneuver based onthe rules used in stage 3200. If stage 3110 is implemented, minimizingthe change in the pitch and roll angles reduces the risk that the wingswould hit the ground. The span of the first part of the substantiallyvertical ascent in which changes in the pitch and roll angles areminimized may be defined by duration (e.g. 1 second), by altitude (e.g.1 meter above initial altitude), by a combination of both (e.g. thefirst of the former thresholds to be crossed), etc. The changeminimization in stage 3110 may be implemented for keeping the wings ofthe air vehicle at a substantially horizontal plane, in pitch and/orroll angles. Yaw may also be maintained during this stage, but this isnot necessarily so.

The rest of the substantially vertical ascent is controlled during stage3120. The controlling of stage 3120 includes controlling aerodynamicparts of the air vehicle for controllably climbing in a substantiallyvertical course. However, in stage 3120, if the air vehicle has somenon-zero (or above another threshold) positive airspeed (i.e., it movesforward), stage 3120 may include controlling the longitudinal airspeedof the vehicle in order to limit reduction of this airspeed (e.g. bymaintaining the airspeed). This is done in order not to waste energy. Ifthe airspeed is low, it is optionally maintained so. Optionally, instage 3120, if the air vehicle has some non-zero groundspeed (or aboveanother threshold), stage 3120 may include controlling the groundspeedof the vehicle in order to limit reduction of this groundspeed (e.g. bymaintaining the airspeed). This is done in order not to waste energy.Once again, if the airspeed is low, it is optionally maintained so.

In stage 3120 (also referred to as “jump up”), the pitch and roll of theair vehicle are controlled and may be changed in order to keep thecourse of the air vehicle (based on its airspeed command, groundspeedcommand, altitude command, RoC command, or any combination of suchcommands).

Optionally, the controlling of the substantially vertical ascent instage 3100 (especially in stage 3120) is based on groundspeed and to alesser extent (or alternatively to no extent) on the airspeed of thevehicle.

Accordingly, stage 3200 may be preceded (or initiated by) a stage ofswitching from a groundspeed based control scheme to an airspeed basedcontrol scheme.

Optionally, during stage 3100, and especially in stage 3110 (alsoreferred to as “jump up”), the controlling of the aerodynamicallyeffective components of the air vehicle is not based on location orchanging of location (speed), but rather only on angles (pitch, yaw,and/or roll).

While the entire substantially vertical ascent of the air vehicle fromthe ground may be executed as discussed with reference to the first partof it, optionally this ascent may also include a second part in whichthe air vehicle still ascends substantially vertically, but iscontrolled by another control scheme, which takes into account speed andlocation, and not only angle. Such a control scheme may be groundspeedbased, airspeed based, or an amalgamation thereof. Especially, while notnecessarily so, the control scheme implemented in such second part ofthe substantially vertical ascent may be the same control scheme usedduring stage 3200 (with an acceleration command of substantially zero,and with a positive altitude/RoC command).

Regarding method 3000 as a whole, it is noted that maximal times fromswitching from stage to stage may be defined. Usually, transition fromone stage to another is executed when some flight-dependent parametersare reached (e.g. when crossing an airspeed threshold, when crossing analtitude/RoC threshold, etc.). If maximal time is defined to suchtransition from one stage to another, and if such thresholds were notmet by the maximal time, the transition may be executed in spite of thatfact, or the stage may be terminated and a landing oriented controlscheme may be implemented instead.

For example, method 3000 may include issuing a positive airspeed commandfor accelerating the air vehicle (i.e., issuing a command for bringingthe air vehicle to a positive airspeed, optionally a defined one, butnot necessarily) if a predetermined time elapsed from an initiation ofthe substantially vertical ascent, regardless of sensors data.

Controlling of the course of the air vehicle during the acceleration ofmethod 3000 may be achieved at least by controlling an operation of oneor more of the aerodynamic subsystems of the air vehicle. Such parts mayinclude, by way of example, the at least one tiltable propulsion unit,at least one non-tiltable propulsion unit, a throttle, an engine,ailerons, elevators, rudder, ruddervator, flaperons, elevons, wingflaps, slats, spoilers, air brakes, variable-sweep wings, non-tiltablepropulsion unit, blades of rotors, and so on. It is noted that differentstages of method 3000 (e.g. the controlling of stage 3100, 3200, and3300, or of any combination thereof) may include controlling anoperation of at least one aerodynamic subsystem of the air vehicleselected from a group consisting of an aileron, an elevator, a rudder, aruddervator, a flaperon, elevons, and a wing flap.

The controlling of the various stages of method 3000 may be executedmanually, automatically and autonomously, and/or by a supervisedcomputer (possibly manipulated by human user input). For example, themodifying of a tilt angle of the tiltable propulsion unit may includeautomated controlling by at least one processor of a control unitmounted on the air vehicle.

As mentioned above, controlling of the operation of the at least onetiltable propulsion unit during the descent is not an easy task, andneither is the controlling of such unit during an acceleration and/orascent of the air vehicle. This may result, by way of example, from theopposing roles it serves and due to the hazardous results that may yieldif controlled erroneously—even when assisted by controlling of otheraerodynamically effective components of the air vehicle, e.g., asdescribed above.

The controlling of the acceleration process may therefore includecontrolling the deceleration process of the air vehicle that includes awing (referring to the examples set forth in the previous drawings, thewing may be wing 320), and balancing between contradictory aerodynamiceffects resulting from the wing and from the at least one tiltablepropulsion unit.

Method 3000 may also include stage 3700 of controlling a horizontalprogression direction of the air vehicle. Optional stage 3700 includescontrolling a horizontal progression direction of the air vehicle. It isnoted that the horizontal progression direction may be keptsubstantially uniform during the entire course taken by the air vehicleduring method 3000, but this is not necessarily so. The controlling ofthe horizontal progression direction may be carried out in response to aset horizontal progression direction—that may be fixed or changed fromtime to time. The determining of such a set horizontal progressiondirection may be based on various considerations—such as geometricconsiderations (e.g. distance from a destination location and currentaltitude), atmospheric conditions (e.g. wind), aerodynamic and/orenergetic efficiency, operational considerations, capabilities of theair vehicle, and so forth. While some parameters may be determined by asystem (or person) implementing method 3000 (e.g. in stage 3900thereof), some parameters may be determined to the system by anothersystem, module, or person. In certain examples, some such parametersthat may be defined before the controlling of method 3000 are asfollows:

-   -   a. Where should the air vehicle be directed? (e.g. what is the        destination of the air vehicle? What is the planned flying        course, during acceleration and or following it?)    -   b. To what height should the air vehicle aim?    -   c. At which direction should the air vehicle fly? (e.g. with the        wind, at an azimuth of 271°, etc.)    -   d. At what angle should the air vehicle ascend?    -   e. What are the horizontal ranges allocated to some or all of        the different sub-stages?    -   f. What are the timing constrains for the acceleration?

While not necessarily all of these parameters are determined in advance,it is possible that all of them (or any sub-combination thereof, e.g.including parameters a, b, c, d, and e but not f, and so forth) aredetermined in advance. The controlling of the different stages (3100,3200, 3300, and 3700) may depend directly on these parameters (or someof them, depending on the stage), but may also depend on parametersdetermined in stage 3900 based on these parameters.

Method 3000 may also include optional stage 3800 of repeatedly checkingwhether flight parameters permit acceleration within predeterminedconstrains. Such repeated checking may be carried out at regular orirregular intervals, and each instance may be triggered by time or basedon one or more measured parameters. The repeated checking may commence,for example, during the first part of the acceleration (e.g. in stage3100) and at least until the tilting of the at least one tiltablepropulsion unit in stage 3300. By way of example, the repeated checkingmay be carried out at least until a first occurrence of an eventselected from (a) receiving a negative result and (b) receivingconfirmation of successful tiling of the tiltable propulsion unit.

If a negative result is received in the checking (i.e. if at least oneof the flight parameters exceeds such an envelope), the method mayinclude either an aborting of the acceleration, but may also includereiterating some parts of the method, re-entering the envelope, andgoing through the sequence of stages to a successful result.

Method 3000 may include optional stage 3900 of determining one or moredesired spatial parameters for the air vehicle.

FIG. 19 illustrates a possible control scheme which may be implementedby system 1201, and/or for executing method 3000, according to anembodiment of the invention.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

It will be appreciated that the embodiments described above are cited byway of example, and various features thereof and combinations of thesefeatures can be varied and modified.

While various embodiments have been shown and described, it will beunderstood that there is no intent to limit the invention by suchdisclosure, but rather, it is intended to cover all modifications andalternate constructions falling within the scope of the invention, asdefined in the appended claims.

1. A control system configured to control an acceleration of an airvehicle which comprises a tiltable propulsion unit that is tiltable toprovide a thrust whose direction is variable at least between a generalvertical thrust vector direction and a general longitudinal thrustvector direction with respect to the air vehicle, the control systemcomprising: an input interface for receiving information indicative of amonitored airspeed of the air vehicle; and a control unit, configured toissue controlling commands to a controller of the tiltable propulsionunit for controlling the acceleration of the air vehicle, wherein thecontrolling of the acceleration comprises: in a first part of theacceleration, in which the tiltable propulsion unit provides thrust inthe general vertical thrust vector direction: (a) controlling anoperation of the at least one tiltable propulsion unit for providinglift to the air vehicle, and (b) controlling a modifying of a tilt angleof the tiltable propulsion unit with respect to a fuselage of the airvehicle, based on: (i) the monitored airspeed and (ii) an airspeedcommand being based on a preplanned acceleration function; and followinga tilting of the at least one tiltable propulsion unit, controlling in asecond part of the acceleration an operation of the tiltable propulsionunit to provide thrust in the general longitudinal thrust vectordirection for propelling the air vehicle.
 2. An air vehicle system,comprising: a wing; a tiltable propulsion unit, tiltable to provide athrust whose direction is variable at least between a general verticalthrust vector direction and a general longitudinal thrust vectordirection with respect to the air vehicle; and a control unit,configured to issue controlling commands to a controller of the tiltablepropulsion unit for controlling the acceleration of the air vehicle,wherein the controlling of the acceleration comprises: in a first partof the acceleration, in which the tiltable propulsion unit providesthrust in the general vertical thrust vector direction: (a) controllingan operation of the at least one tiltable propulsion unit for providinglift to the air vehicle, and (b) controlling a modifying of a tilt angleof the tiltable propulsion unit with respect to a fuselage of the airvehicle, based on: (i) a monitored airspeed of the air vehicle and (ii)an airspeed command being based on a preplanned acceleration function;and following a tilting of the at least one tiltable propulsion unit,controlling in a second part of the acceleration an operation of thetiltable propulsion unit to provide thrust in the general longitudinalthrust vector direction for propelling the air vehicle.
 3. The systemaccording to claim 1, wherein the control unit is configured to controlthe operation of the tiltable propulsion unit automatically.
 4. Thesystem according to claim 1, wherein the control unit comprises a pitchcontrol module which is configured to issue a pitch command based on themonitored airspeed and the airspeed command and to control in the firstpart of the acceleration a modifying of a pitch angle of the air vehiclebased on the pitch command, wherein the controlling of the modifying ofthe tilt angle is further based on the pitch command.
 5. The systemaccording to claim 4, wherein the control unit is configured to issue arotor tilt command for lowering a rotor tilt angle as a result of anissue of a pitch command for lowering a pitch angle.
 6. The systemaccording to claim 4, wherein the pitch control module is configured toissue the pitch command further in response to the measured altitude ofthe air vehicle.
 7. The system according to claim 4, wherein the controlunit is configured to change a ratio between: (a) a contribution of thetiltable propulsion unit to the modification of the pitch and (b) acontribution of at least one elevator to the modification of the pitch,wherein the changing of the ratio is correlated to the monitoredairspeed.
 8. The system according to claim 1, wherein the control unitis further configured to control thrust power of the tiltable propulsionunit for reducing a difference between the measured airspeed and a setairspeed, while restricting increasing of the thrust power based on athreshold that is determined in response to the measured airspeed. 9.The system according to claim 1, wherein the control unit comprises atilting control module that is configured to determine a timing fortilting of the at least one tiltable propulsion unit between the secondand the first parts of the acceleration for minimizing a duration inwhich the tiltable propulsion unit provides thrust in the generalvertical thrust vector direction.
 10. The system according to claim 1,wherein the pitch control module is further configured to keep a pitchof the air vehicle during at least a part of the first part of theacceleration within a permitted pitch range that is dynamicallydetermined in response to the measured airspeed of the air vehicle. 11.The system according to claim 1, wherein the control unit is configuredto control before the first part of the acceleration a substantiallyvertical ascent of the air vehicle from the ground, and to controlaerodynamic parts of the air vehicle in a first part of thesubstantially vertical ascent from the ground for minimizing change inpitch angle and roll angle of the air vehicle.
 12. The system accordingto claim 11, wherein the control unit is configured to issue a positiveairspeed command for accelerating the air vehicle if predetermined timeelapsed from an initiation of the substantially vertical ascent,regardless of sensors data.
 13. The system according to claim 1, furthercomprising a pitch control module which is configured to control in thefirst part of the acceleration a modifying of a pitch angle of the airvehicle, wherein the control unit is configured to balance betweenmodifying the tilt angle and modifying the pitch angle for reducing anenergy cost of the acceleration of the air vehicle.
 14. The systemaccording to claim 1, further comprising a pitch control module which isconfigured to control in the first part of the acceleration a modifyingof a pitch angle of the air vehicle, and to control in the first part ofthe acceleration a modifying of a pitch angle of the air vehicle,thereby preventing creation of negative lift force by the wing.
 15. Thesystem according to claim 1, further comprising a pitch control modulewhich is configured to control in the first part of the acceleration amodifying of a pitch angle of the air vehicle and to limit in the firstpart of the acceleration lift created by the wing below 80% of maximallift which may be created by the wing in the monitored airspeed.
 16. Thesystem according to claim 1, wherein the control unit is configured tocontrol thrust power of the tiltable propulsion unit for reducing adifference between the measured airspeed and a set airspeed, whilerestricting reduction of the thrust power based on a lower thresholdthat is determined in response to a measured airspeed of the airvehicle.
 17. The system according to claim 16, further comprising analtitude control module configured to minimize a vertical deviation ofthe air vehicle from a set altitude during the first part of theacceleration, wherein the minimizing is restricted at least by therestricting of the reduction of the thrust power based on the lowerthreshold.
 18. A method for controlling an acceleration of an airvehicle which comprises a tiltable propulsion unit that is tiltable toprovide a thrust whose direction is variable at least between a generalvertical thrust vector direction and a general longitudinal thrustvector direction with respect to the air vehicle, the method comprising:in a first part of the acceleration, in which the tiltable propulsionunit provides thrust in the general vertical thrust vector direction:(a) controlling an operation of the at least one tiltable propulsionunit for providing lift to the air vehicle, and (b) controlling amodifying of a tilt angle of the tiltable propulsion unit with respectto a fuselage of the air vehicle, based on: (i) the monitored airspeedand (ii) an airspeed command being based on a preplanned accelerationfunction; and following a tilting of the at least one tiltablepropulsion unit, controlling in a second part of the acceleration anoperation of the tiltable propulsion unit to provide thrust in thegeneral longitudinal thrust vector direction for propelling the airvehicle.
 19. The method according to claim 18, wherein the controllingof the modifying of a tilt angle of the tiltable propulsion unitcomprises automated controlling by at least one processor of a controlunit mounted on the air vehicle.
 20. The method according to claim 18,wherein the controlling of the acceleration of the air vehicle comprisescontrolling the acceleration of the air vehicle that comprises a wing,and balancing between contradictory aerodynamic effects resulting fromthe wing and from the at least one tiltable propulsion unit.
 21. Themethod according to claim 18, further comprising issuing a pitch commandbased on the monitored airspeed and the airspeed command and controllingin the first part of the acceleration a modifying of a pitch angle ofthe air vehicle based on the pitch command, wherein the controlling ofthe modifying of the tilt angle is further based on the pitch command.22. The method according to claim 21, wherein the controlling of themodifying of the tilt angle comprises issuing a rotor tilt command forlowering a rotor tilt angle as a result of an issue of a pitch commandfor lowering a pitch angle.
 23. The method according to claim 21,wherein the issuing of the pitch command is further based on themeasured altitude of the air vehicle.
 24. The method according to claim18, comprising controlling thrust power of the tiltable propulsion unitfor reducing a difference between the measured airspeed and a setairspeed, while restricting increasing of the thrust power based on athreshold that is determined in response to the measured airspeed. 25.The method according to claim 18, further comprising determining timingfor tilting of the at least one tiltable propulsion unit between thesecond and the first parts of the acceleration for minimizing a durationin which the tiltable propulsion unit provides thrust in the generalvertical thrust vector direction.
 26. The method according to claim 18,further comprising keeping a pitch of the air vehicle during at least apart of the first part of the acceleration within a permitted pitchrange that is dynamically determined in response to the measuredairspeed of the air vehicle.
 27. The method according to claim 26,wherein the keeping of the pitch within the permitted pitch rangeprevents stalling of the air vehicle.
 28. The method according to claim18, further comprising controlling before the first part of theacceleration a substantially vertical ascent of the air vehicle from theground, and in a first part of the substantially vertical ascent fromthe ground controlling aerodynamic parts of the air vehicle forminimizing change in pitch angle and roll angle of the air vehicle. 29.The method according to claim 28, comprising issuing a positive airspeedcommand for accelerating the air vehicle if predetermined time elapsedfrom an initiation of the substantially vertical ascent, regardless ofsensors data.
 30. The method according to claim 18, further comprisingcontrolling in the first part of the acceleration a modifying of a pitchangle of the air vehicle, and balancing between modifying the tilt angleand modifying the pitch angle for reducing an energy cost of theacceleration of the air vehicle.
 31. The method according to claim 18,further comprising controlling in the first part of the acceleration amodifying of a pitch angle of the air vehicle, thereby preventingcreation of negative lift force by the wing.
 32. The method according toclaim 18, further comprising controlling in the first part of theacceleration a modifying of a pitch angle of the air vehicle, therebylimiting lift created by the wing below 80% of maximal lift which may becreated by the wing in the monitored airspeed.
 33. The method accordingto claim 18, wherein at least the controlling of the acceleratingcomprises controlling an operation of at least one aerodynamic part ofthe air vehicle selected from a group consisting of an aileron, anelevator, a rudder, a ruddervator, a flaperon, elevons, and a wing flap.34. The method according to claim 18, further comprising controllingthrust power of the tiltable propulsion unit for reducing a differencebetween the measured airspeed and a set airspeed, while restrictingreduction of the thrust power based on a lower threshold that isdetermined in response to a measured airspeed of the air vehicle. 35.The method according to claim 34, further comprising minimizing avertical deviation of the air vehicle from a set altitude during thefirst part of the acceleration, wherein the minimizing is restricted atleast by the restricting of the reduction of the thrust power based onthe lower threshold.
 36. A program storage device readable by machine,tangibly embodying a computer readable code portion executable by themachine for controlling an acceleration of an air vehicle whichcomprises a tiltable propulsion unit that is tiltable to provide athrust whose direction is variable at least between a general verticalthrust vector direction and a general longitudinal thrust vectordirection with respect to the air vehicle, the computer readable codeportion comprising instructions for: in a first part of theacceleration, in which the tiltable propulsion unit provides thrust inthe general vertical thrust vector direction: (a) controlling anoperation of the at least one tiltable propulsion unit for providinglift to the air vehicle, and (b) controlling a modifying of a tilt angleof the tiltable propulsion unit with respect to a fuselage of the airvehicle, based on: (i) the monitored airspeed and (ii) an airspeedcommand being based on a preplanned acceleration function; and followinga tilting of the at least one tiltable propulsion unit, controlling in asecond part of the acceleration an operation of the tiltable propulsionunit to provide thrust in the general longitudinal thrust vectordirection for propelling the air vehicle.
 37. The system according toclaim 2, wherein the control unit is configured to control the operationof the tiltable propulsion unit automatically.
 38. The system accordingto claim 2, wherein the control unit comprises a pitch control modulewhich is configured to issue a pitch command based on the monitoredairspeed and the airspeed command and to control in the first part ofthe acceleration a modifying of a pitch angle of the air vehicle basedon the pitch command, wherein the controlling of the modifying of thetilt angle is further based on the pitch command.
 39. The systemaccording to claim 38, wherein the control unit is configured to issue arotor tilt command for lowering a rotor tilt angle as a result of anissue of a pitch command for lowering a pitch angle.
 40. The systemaccording to claim 38, wherein the pitch control module is configured toissue the pitch command further in response to the measured altitude ofthe air vehicle.
 41. The system according to claim 38, wherein thecontrol unit is configured to change a ratio between: (a) a contributionof the tiltable propulsion unit to the modification of the pitch and (b)a contribution of at least one elevator to the modification of thepitch, wherein the changing of the ratio is correlated to the monitoredairspeed.
 42. The system according to claim 2, wherein the control unitis further configured to control thrust power of the tiltable propulsionunit for reducing a difference between the measured airspeed and a setairspeed, while restricting increasing of the thrust power based on athreshold that is determined in response to the measured airspeed. 43.The system according to claim 2, wherein the control unit comprises atilting control module that is configured to determine a timing fortilting of the at least one tiltable propulsion unit between the secondand the first parts of the acceleration for minimizing a duration inwhich the tiltable propulsion unit provides thrust in the generalvertical thrust vector direction.
 44. The system according to claim 2,wherein the pitch control module is further configured to keep a pitchof the air vehicle during at least a part of the first part of theacceleration within a permitted pitch range that is dynamicallydetermined in response to the measured airspeed of the air vehicle. 45.The system according to claim 2, wherein the control unit is configuredto control before the first part of the acceleration a substantiallyvertical ascent of the air vehicle from the ground, and to controlaerodynamic parts of the air vehicle in a first part of thesubstantially vertical ascent from the ground for minimizing change inpitch angle and roll angle of the air vehicle.
 46. The system accordingto claim 45, wherein the control unit is configured to issue a positiveairspeed command for accelerating the air vehicle if predetermined timeelapsed from an initiation of the substantially vertical ascent,regardless of sensors data.
 47. The system according to claim 2, furthercomprising a pitch control module which is configured to control in thefirst part of the acceleration a modifying of a pitch angle of the airvehicle, wherein the control unit is configured to balance betweenmodifying the tilt angle and modifying the pitch angle for reducing anenergy cost of the acceleration of the air vehicle.
 48. The systemaccording to claim 2, further comprising a pitch control module which isconfigured to control in the first part of the acceleration a modifyingof a pitch angle of the air vehicle, and to control in the first part ofthe acceleration a modifying of a pitch angle of the air vehicle,thereby preventing creation of negative lift force by the wing.
 49. Thesystem according to claim 2, further comprising a pitch control modulewhich is configured to control in the first part of the acceleration amodifying of a pitch angle of the air vehicle and to limit in the firstpart of the acceleration lift created by the wing below 80% of maximallift which may be created by the wing in the monitored airspeed.
 50. Thesystem according to claim 2, wherein the control unit is configured tocontrol thrust power of the tiltable propulsion unit for reducing adifference between the measured airspeed and a set airspeed, whilerestricting reduction of the thrust power based on a lower thresholdthat is determined in response to a measured airspeed of the airvehicle.
 51. The system according to claim 50, further comprising analtitude control module configured to minimize a vertical deviation ofthe air vehicle from a set altitude during the first part of theacceleration, wherein the minimizing is restricted at least by therestricting of the reduction of the thrust power based on the lowerthreshold.